Please do me a favor, a math problem, with equation solution! For a job, it takes 12 days for a to do it alone, and 20 days for B to do it alone. Now, after a few hours, the rest is done by B, and B completes it. If B does 4 days more than a, how many days has a done?

Please do me a favor, a math problem, with equation solution! For a job, it takes 12 days for a to do it alone, and 20 days for B to do it alone. Now, after a few hours, the rest is done by B, and B completes it. If B does 4 days more than a, how many days has a done?

Let a do X days, then B do X + 4 days
x/12+(x+4)/20=1 8x/60+1/5=1 8x=48 x=6

How to write the corresponding data after writing the equation
Specifically refers to the calculation of a good equation, do not know the corresponding data should be written in which material below, the key point is specific

Chemical reaction
1. Combustion of magnesium in air: 2mg + O2 ignites 2MgO
2. Combustion of iron in oxygen: 3Fe + 2O2 ignites Fe3O4
3. Combustion of aluminum in air: 4Al + 3O2 ignites 2al2o3
4. Combustion of hydrogen in air: 2h2 + O2 ignites 2H2O
5. Combustion of red phosphorus in air: 4P + 5o2 ignites 2p2o5
6. Sulfur powder burning in air: S + O2 ignites SO2
7. Full combustion of carbon in oxygen: C + O2 ignites CO2
8. Incomplete combustion of carbon in oxygen: 2C + O2 ignites 2CO
9. Carbon dioxide passing through hot carbon layer: C + CO2 high temperature 2CO
10. Combustion of carbon monoxide in oxygen: 2CO + O2 ignites 2co2
11. Reaction of carbon dioxide and water (carbon dioxide into purple litmus test solution): CO2 + H2O = = = H2CO3
12. Quicklime dissolved in water: Cao + H2O = = = Ca (OH) 2
13. Anhydrous copper sulfate as desiccant: CuSO4 + 5H2O = = = = CuSO4 · 5H2O
14. Combustion of sodium in chlorine: 2Na + Cl2 ignites 2nacl
decomposition reaction
15. Preparation of oxygen from hydrogen peroxide in laboratory: 2h2o2, MnO2, 2H2O + O2 ↑
16. Heating potassium permanganate: 2kmno4 heating k2mno4 + MnO2 + O2 ↑
17. Under the action of direct current, water is electrified by 2H2O, 2h2 ↑ + O2 ↑
18. The carbonic acid is unstable and separated into H2CO3 = = = H2O + CO2 ↑
19. High temperature calcined limestone (carbon dioxide industrial process): CaCO3 high temperature CaO + CO2 ↑
Displacement reaction
20. Reaction of iron with copper sulfate solution: Fe + CuSO4 = = FeSO4 + Cu
21. Reaction of zinc with dilute sulfuric acid (hydrogen production in laboratory): Zn + H2SO4 = = ZnSO4 + H2 ↑
22. Reaction of magnesium with dilute hydrochloric acid: Mg + 2HCl = = = MgCl2 + H2 ↑
23. Reduction of copper oxide by hydrogen: H2 + CuO heating Cu + H2O
24. Reduction of CuO by charcoal: C + 2CuO, high temperature 2Cu + CO2 ↑
25. Methane burning in air: CH4 + 2O2 ignites CO2 + 2H2O
26. Water vapor passes through the hot carbon layer: H2O + C, high temperature H2 + Co
27. Coke reduced iron oxide: 3C + 2fe2o3, high temperature 4Fe + 3co2 ↑
other
28. Reaction of sodium hydroxide solution with copper sulfate solution: 2naoh + CuSO4 = = Cu (OH) 2 ↓ + Na2SO4
29. Methane burning in air: CH4 + 2O2 ignites CO2 + 2H2O
30. Alcohol burning in air: C2H5OH + 3O2 ignites 2co2 + 3H2O
31. Reduction of copper oxide by carbon monoxide: CO + CuO heating Cu + CO2
32. Carbon monoxide reduced iron oxide: 3Co + Fe2O3, high temperature 2Fe + 3co2
33. Carbon dioxide: Ca (OH) 2 + CO2 = = = = CaCO3 ↓ + H2O
34. Reaction of sodium hydroxide with carbon dioxide (removal of carbon dioxide): 2naoh + CO2 = = = = Na2CO3 + H2O
35. Reaction of limestone (or marble) with dilute hydrochloric acid (laboratory preparation of carbon dioxide): CaCO3 + 2HCl = = = CaCl2 + H2O + CO2 ↑
36, sodium carbonate and concentrated hydrochloric acid reaction (foam extinguisher principle): Na2CO3 + 2HCl = = = 2NaCl + H2O + CO2
1、 Reaction of substance with oxygen:
(1) The reaction of elemental matter with oxygen is as follows
1. Combustion of magnesium in air: 2mg + O2 ignites 2MgO
2. Combustion of iron in oxygen: 3Fe + 2O2 ignites Fe3O4
3. Copper heated in air: 2Cu + O2 heated 2CuO
4. Combustion of aluminum in air: 4Al + 3O2 ignites 2al2o3
5. Combustion in hydrogen and air: 2h2 + O2 ignites 2H2O
6. Combustion of red phosphorus in air: 4P + 5o2 ignites 2p2o5
7. Sulfur powder burning in air: S + O2 ignites SO2
8. Full combustion of carbon in oxygen: C + O2 ignites CO2
9. Incomplete combustion of carbon in oxygen: 2C + O2 ignites 2CO
(2) Reaction of compound with oxygen:
10. Combustion of carbon monoxide in oxygen: 2CO + O2 ignites 2co2
11. Methane combustion in air: CH4 + 2O2 ignites CO2 + 2H2O
12. Alcohol burning in air: C2H5OH + 3O2 ignites 2co2 + 3H2O
2、 There are several decomposition reactions
13. Under the action of direct current, water is electrified by 2H2O, 2h2 ↑ + O2 ↑
14. Heating basic copper carbonate: Cu2 (OH) 2CO3 heating 2CuO + H2O + CO2 ↑
15. Heating potassium chlorate (with a small amount of manganese dioxide): 2kclo3 = = = = 2KCL + 3O2 ↑
16. Heating potassium permanganate: 2kmno4 heating k2mno4 + MnO2 + O2 ↑
17. Carbonic acid is not stable, but H2CO3 = = = H2O + CO2 ↑
18. High temperature calcined limestone: CaCO3, high temperature CaO + CO2 ↑
3、 There are several redox reactions
19. Reduction of CuO by hydrogen: H2 + CuO heating Cu + H2O
20. Reduction of CuO by charcoal: C + 2CuO, high temperature 2Cu + CO2 ↑
21. Reduction of iron oxide by coke: 3C + 2fe2o3, high temperature 4Fe + 3co2 ↑
22. Reduction of Fe3O4 by coke: 2C + Fe3O4, high temperature 3Fe + 2co2 ↑
23. Reduction of copper oxide by carbon monoxide: CO + CuO heating Cu + CO2
24. Reduction of iron oxide by carbon monoxide: 3Co + Fe2O3, high temperature 2Fe + 3co2
25. Reduction of Fe3O4 by carbon monoxide: 4Co + Fe3O4, high temperature 3Fe + 4co2
4、 The relationship among element, oxide, acid, alkali and salt
(1) Metal element + acid -------- Salt + hydrogen (displacement reaction)
26. Zinc and dilute sulfuric acid Zn + H2SO4 = ZnSO4 + H2 ↑
27. Iron and dilute sulfuric acid Fe + H2SO4 = FeSO4 + H2 ↑
28. Magnesium and dilute sulfuric acid Mg + H2SO4 = MgSO4 + H2 ↑
29. Aluminum and dilute sulfuric acid 2Al + 3h2so4 = Al2 (SO4) 3 + 3h2 ↑
30. Zinc and dilute hydrochloric acid Zn + 2HCl = = = ZnCl2 + H2 ↑
31. Iron and dilute hydrochloric acid Fe + 2HCl = = = FeCl2 + H2 ↑
32. Magnesium and dilute hydrochloric acid Mg + 2HCl = = = MgCl2 + H2 ↑
33. Aluminum and dilute hydrochloric acid 2Al + 6hcl = = 2alcl3 + 3h2 ↑
(2) Metal element + salt (solution) -- another metal + another salt
34. Reaction between iron and copper sulfate solution: Fe + CuSO4 = = = FeSO4 + Cu
35. Reaction between zinc and copper sulfate solution: Zn + CuSO4 = = = ZnSO4 + Cu
36. Reaction of copper with mercuric nitrate solution: Cu + Hg (NO3) 2 = = = Cu (NO3) 2 + Hg
(3) Alkaline oxide + acid -------- Salt + water
37. Reaction of iron oxide with dilute hydrochloric acid: Fe2O3 + 6hcl = = = 2fecl3 + 3H2O
38. Reaction of iron oxide with dilute sulfuric acid: Fe2O3 + 3h2so4 = = = Fe2 (SO4) 3 + 3H2O
39. Reaction of copper oxide with dilute hydrochloric acid: CuO + 2HCl = = = = CuCl2 + H2O
40. Reaction of copper oxide with dilute sulfuric acid: CuO + H2SO4 = = = = CuSO4 + H2O
41. Reaction of magnesium oxide with dilute sulfuric acid: MgO + H2SO4 = = = = MgSO4 + H2O
42. Reaction of calcium oxide with dilute hydrochloric acid: Cao + 2HCl = = = = CaCl2 + H2O
(4) Acid oxide + alkali salt + water
43. Deterioration of caustic soda exposed to air: 2naoh + CO2 = = = = Na2CO3 + H2O
44. Caustic soda absorbs sulfur dioxide: 2naoh + SO2 = = = = Na2SO3 + H2O
45. Caustic soda absorbs sulfur trioxide gas: 2naoh + SO3 = = = = Na2SO4 + H2O
46. Slaked lime metamorphoses in air: Ca (OH) 2 + CO2 = = = = CaCO3 ↓ + H2O
47. Absorption of sulfur dioxide by hydrated lime: Ca (OH) 2 + SO2 = = = = CaSO3 ↓ + H2O
(5) Acid + alkali salt + water
48. Hydrochloric acid reacts with caustic soda: HCl + NaOH = = = = NaCl + H2O
49. Reaction of hydrochloric acid and potassium hydroxide: HCl + KOH = = = = KCl + H2O
50. Reaction of hydrochloric acid and copper hydroxide: 2HCl + Cu (OH) 2 = = = = CuCl2 + 2H2O
51. Reaction of hydrochloric acid and calcium hydroxide: 2HCl + Ca (OH) 2 = = = = CaCl2 + 2H2O
52. Reaction of hydrochloric acid and ferric hydroxide: 3hcl + Fe (OH) 3 = = = = FeCl3 + 3H2O
53. Aluminum hydroxide in the treatment of gastric acidosis: 3hcl + Al (OH) 3 = = = = AlCl3 + 3H2O
54. Reaction of sulfuric acid and caustic soda: H2SO4 + 2naoh = = = = Na2SO4 + 2H2O
55. Reaction of sulfuric acid and potassium hydroxide: H2SO4 + 2koh = = = K2SO4 + 2H2O
56. Reaction of sulfuric acid and copper hydroxide: H2SO4 + Cu (OH) 2 = = = = CuSO4 + 2H2O
57. Reaction of sulfuric acid and ferric hydroxide: 3h2so4 + 2Fe (OH) 3 = = = Fe2 (SO4) 3 + 6H2O
58. Reaction of nitric acid and caustic soda: HNO3 + NaOH = = = = NaNO3 + H2O
(6) Acid + salt --- another acid + another salt
59. Reaction of marble with dilute hydrochloric acid: CaCO3 + 2HCl = = = CaCl2 + H2O + CO2 ↑
60. Reaction of sodium carbonate with dilute hydrochloric acid: Na2CO3 + 2HCl = = = 2nacl + H2O + CO2 ↑
61. Reaction of magnesium carbonate with dilute hydrochloric acid: MgCO3 + 2HCl = = = MgCl2 + H2O + CO2 ↑
62. Reaction of hydrochloric acid and silver nitrate solution: HCl + AgNO3 = = = AgCl ↓ + HNO3
63. Reaction of sulfuric acid and sodium carbonate: Na2CO3 + H2SO4 = = = Na2SO4 + H2O + CO2 ↑
64. Reaction of sulfuric acid and barium chloride solution: H2SO4 + BaCl2 = = = = BaSO4 ↓ + 2HCl
(7) Alkali + salt --- another alkali + another salt
65. Sodium hydroxide and copper sulfate: 2naoh + CuSO4 = = = = Cu (OH) 2 ↓ + Na2SO4
66. Sodium hydroxide and ferric chloride: 3naoh + FeCl3 = = = = Fe (OH) 3 ↓ + 3nacl
67. Sodium hydroxide and magnesium chloride: 2naoh + MgCl2 = = = = Mg (OH) 2 ↓ + 2nacl
68. Sodium hydroxide and copper chloride: 2naoh + CuCl2 = = = = Cu (OH) 2 ↓ + 2nacl
69. Calcium hydroxide and sodium carbonate: Ca (OH) 2 + Na2CO3 = = = CaCO3 ↓ + 2naoh
(8) Salt + salt -- two new salts
70. Sodium chloride solution and silver nitrate solution: NaCl + AgNO3 = = = = AgCl ↓ + NaNO3
71. Sodium sulfate and barium chloride: Na2SO4 + BaCl2 = = = BaSO4 ↓ + 2nacl
5、 Other reactions:
72. CO2 dissolved in water: CO2 + H2O = = = H2CO3
73. Quicklime dissolves in water: Cao + H2O = = = Ca (OH) 2
74. Sodium oxide dissolves in water: Na2O + H2O = = = = 2naoh
75. Sulfur trioxide dissolved in water: SO3 + H2O = = = = H2SO4
76. Heating CuSO4 · 5H2O, heating CuSO4 + 5H2O
77. Anhydrous copper sulfate as desiccant: CuSO4 + 5H2O = = = = CuSO4 · 5h2
Application of chemical equation reaction phenomenon
2mg + O2 ignites or Δ 2MgO burns violently. It produces bright white light, white solid, exothermic and a lot of white smoke and white signal bombs
Experiment on ignition of silver white liquid or Δ 2hgo by 2hg + O2 to form red solid lavasside
2Cu + O2 ignites or Δ 2CuO red metal becomes black solid
4Al + 3O2 ignites or Δ 2al2o3 silver white metal becomes white solid
3Fe + 2O2 ignites Fe3O4 to burn violently, Mars radiates all around, forms black solid, exothermic 4Fe + 3O2 and high temperature 2fe2o3
C + O2 ignites CO2 to burn violently, white light, release heat and make lime water turbid
S + O2 ignites SO 2 intense combustion, exothermic, pungent gas, light blue flame in air, and blue purple flame in oxygen
2h2 + O2 ignites the light blue flame of 2H2O, releases heat, and forms the liquid (water) high energy fuel which makes the anhydrous CuSO4 turn blue
4P + 5o2 ignites 2p2o5, which burns violently, gives off a lot of white smoke, and produces white solid, which proves the oxygen content in the air
CH4 + 2O2 ignites the blue flame of 2H2O + CO2, releases heat, and produces liquid (water) methane and natural gas which make lime water turbid and anhydrous CuSO4 blue
2c2h2 + 5o2 ignites the blue flame of 2H2O + 4co2, releases heat and black smoke, generates the liquid oxyacetylene flame which makes the lime water turbid and the anhydrous CuSO4 blue, and cuts the metal by welding
2kclo3mno2 Δ 2KCL + 3O2 ↑ generates the gas that reburnies the wood strips with Mars, and oxygen is prepared in laboratory
2kmno4 Δ k2mno4 + MnO2 + O2 ↑ turns purple to black, and produces the gas which makes the wood with Mars reburning. Oxygen is prepared in laboratory
The experimental results show that 2hgo Δ 2hg + O2 ↑ red changes to silvery white, and the formation of gas makes the wood with Mars reburning
When 2H2O is electrified, 2h2 ↑ + O2 ↑ water is electrified and decomposed into hydrogen and oxygen electrolyzed water
Cu2 (OH) 2CO3 Δ 2CuO + H2O + CO2 ↑ green turns black, there is liquid on the wall of the test tube, the limestone water becomes turbid, and the gas is heated by copper green

How to learn the formula format of material quantity calculation problem
Our teacher didn't tell me how to write the answer. I can't understand why each line corresponds to the top and bottom instead of calculating the mass by the relative molecular mass. Who told me about the application of the equation? I'm still in the stage of calculating the mass by the equation first and then the mass by the mass