( 1-20%)( 1+x)= 1
80% + 80%x = 1
80%x = 1 - 80%
80%x = 20%
x = 20% ÷ 80%
x = 25%
A: The sales volume will increase by 25% over the original price.
12. 1) solution: let the per capita quota be x pieces.
(4x+20)/4=(6x-20)/5
5(4x+20)=4(6x-20)
20x+ 100=24x-80
-4x=- 180
x=45
A: The per capita quota this month is 45 yuan.
2) Solution: Let the per capita quota of this month be y pieces.
(4y+20)/4=2+(6y-20)/5
5(4y+20)=40+4(6y-20)
20y- 100=40+24y-80
-4y=- 140
y=35
A: The per capita quota this month is 35 yuan.
3) Solution: Assume that the per capita quota this month is m pieces.
(4m+20)/4=-2+(6m-20)5
5(4m+20)=-40+4(6m-20)
20m+ 100=-40+24m-80
-4m=-220
m=55
A: The per capita quota this month is 55 yuan.
13. 1) solution: make him ***x years old.
1/6x+ 1/ 12x+ 1/7x+5+ 1/2x+4 = x
x=84
A: He is 84 years old.
2)84×( 1/6+ 1/ 12+ 1/7)+5
=33+5
=38 years old
A: He became a father at the age of 38.
3)84-4=80 (years old)
He was 80 when his son died.