Put the ratios into fraction form. Since you do not know how many problems, replace it with an unknown number a.k.a. a variable.
10/30 = x/60
Since you can get 30 to 60 by multiplying by 2, multiply everything on the left side by 2.
Then, you get 20/60, which means 20 = x. So, he can complete 20 minutes an hour.