Gmat prep speed question

The easiest way to do this one is to plug in a number for x.

Let's say that he drove 100 miles, so x = 100.

If he drove 100 miles:

5 miles at 30 mph = 10 minutes
95 miles at 60 mph = 95 minutes
Total = 105 minutes

Now, if he had gone 100 miles at 60 mph, that'd be 100 minutes. So, he was 5 minutes slower, or 5%.

Plug in 100 for x in the answers, and look for the one that is 5%, or answer choice E.