Fred completed 10 math problems. This is 40% of the number of math problems he has to do. How many math problems must he do?

25 math problems

Step-by-step explanation:

He completed 10 out of x, or the total he has to do.

10/x

We know that this equals 40%

10/x=40/100

Cross multiply to solve

40*x=100*10

40x=1000

Divide both sides by 40

x=25

He must do 25 math problems

25 problems

Step-by-step explanation:

10 problems = 40%

1 problem = 4 %

5 problems = 20%

5 problems (20%) x 5 (to get 100%) = 25 problems