Well, we know that he/she can solve 30 questions in 2 minutes. So, we have to find out how many he/she can solve in 1 minute. So:

= 15.
They can solve 15 questions in 1 minute. Now, in order to find how many can be solved in 20 minutes we simply multiply 15 and 20:
15 x 20 = 300.
Sam can solve 300 questions in 20 minutes.