There are 60 minutes in an hour.  

For every 3-minute period in an hour he can do x math problems.

Let's find out how many 3-minute periods there are in 1 hour.

60÷3 is 20, so there are 20 3-minute periods in 1 hour.  

In each of those 20 3-minute periods Joey can do x math problems.

Therefore in all 20 of those 3-minute periods he can do 20x math problems.

So in 1 hour he can do 20x math problems.

Therefore in N hours he can do 20xN math problems.

Edwin