13. When I increase a certain number by $20 \%$, I get twice as much as when I decrease 20 less than the number by $20 \%$. What is that number?
A 40
B 60
С 80
D 100
E 120
13. C Let the certain number be $x$. Then $\frac{6 x}{5}=2 \times \frac{4}{5}(x-20)$. Therefore $6 x=8(x-20)=8 x-160$. Hence $2 x=160$, so $x=80$.