Charles finished 7 laps around the track. This is 20% of the number of laps he has to do. How many laps must he do?

To solve this problem, you have to use a proportion:

7/20 = x/100

After setting up the proportion, you have to cross multiply:

20x=700

Now, solve the equation by dividing by 20 on both sides:

x=35

So Charles has 35 laps left.