(1 / 30 + 1 / 35 + 1 / 63) * 2 / 7

(1 / 30 + 1 / 35 + 1 / 63) * 2 / 7

(1/30+1/35+1/63)*15/7
=(13/210+1/63)*15/7
=49/630*15/7
=1/6
that's OK
Or multiply each by 15 / 7

Simple calculation of 8 × 6.5 + 3 and 1 / 2 × 8-46.5

8 × 6.5 + one third and one half × 8-46.5
=8 × (6.5 + 1 / 3) - 46.5
=8×10-46.5
=80-46.5
=33.5

4.72-3.98 + 5.28-2?

4.72-3.98+5.28-2= (4.72+5.28)-（3.98+2）
=10-5.98
=3.02
Idea: the sum of 4.72 + 5.28 is an integer, and the sum is 10. If (4.72 + 5.28) - 3.98-2, it will take one more step and can not achieve the purpose of simple calculation
I know: in the mixed operation of addition and subtraction, we must first find out the number whose result is an integer and observe how to calculate it more easily
To the questioner: Mathematics needs observation to facilitate our life

A simple calculation method of 14.28 - (10-2.72)
emergency

14.28-(10-2.72)
=14.28-10+2.72
=14.28+2.72-10
=17-10
=7

Simple calculation of 5 × 7.28 square to 5 × 2.72 square

5 × 7.28m2-5 × 2.72m2
=5 × (7.28m2-2.72m2)
=5×(7.28+2.72）×（7.28-2.72）
=5×10×4.56
=50×4.56
=228

Simple calculation of 4.72-3.98 + 5.28-2.02

4.72-3.98+5.28-2.02
=4.72+5.28-（3.98+2.02）
=10-6
=4

1-1 / 2 + 1 / 6 + 1 / 12 + 1 / 20 + 1 / 30 + 1 / 42 + 1 / 36

1-1/2+1/6+1/12+1/20+1/30+1/42+1/56
=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8
=1-1/8
=7/8

Simple calculation of 31 × 870 + 13 × 310

31x870+13x310
=31x870+31x130
=31x(870+130)
=31x1000
=31000

Simple calculation of 87 * 98

87*98
=87*(100-2)
=87*100-87*2
=8700-174
=8526

The product of 2.5 times 1.8 plus 2 times x equals 40, and we find X

2.5*1.8+2x=40
2x=40-2.5*1.8
2x=35.5
x=35.5÷2
x=17.75