Math problems in grade five of primary school Processing a batch of parts, apprentice alone 9 hours to complete, master alone 6 hours to complete (3) Master and apprentice work together for several hours? (please teach me patiently)

Math problems in grade five of primary school Processing a batch of parts, apprentice alone 9 hours to complete, master alone 6 hours to complete (3) Master and apprentice work together for several hours? (please teach me patiently)

The apprentice does 1 / 9 every hour
The master makes 1 / 6 every hour
They do 1 / 9 + 1 / 6 = 5 / 18 per hour
So it takes 1 / (5 / 18) = 18 / 5 = 3.6 hours to finish
That is 3 hours 36 minutes

How much money do a and B each have? 60% of them are from A. if 15 yuan is given to a, the remaining money of B is 3 yuan less than 25% of the total. How much money do a and B have? Explain why!)
The solution to the equation needs to write the process (that is, how to calculate x). My formula is right, but the answer is different Here's my list: 0.4x-15 = 0.25x-3

Let a and B have a total of X Yuan, then a has 60% X Yuan, B has 40% X Yuan. Then we can get the equation: 40% X-15 = 25% x-3 (B reduces 15 yuan, which is equal to 25% of the total amount of money is less than 3 yuan). The solution is x = 80. So a has: 80 x 60% = 48 (yuan) B has: 80 x 40% = 32 (yuan). A: A and B have 48 yuan and 32 yuan respectively

Primary school fifth grade mathematics problem, seek detailed solution, thank you!
Four kinds of milk powder were delivered from non-staple food stores, including 12 cases of type A and type B milk powder, 16 cases of type B and type C milk powder, and 15 cases of type C and type D milk powder?

There are 12 boxes of a and B, 15 boxes of C and D. so the total number is: 12 + 15 = 27 (boxes)
B and C 16 cases, accounting for 16 / 27 of the total