So if he drives 50 miles in 1 hour, then he must drive 100 miles in 2 hours. 0.25 is another way of writing 1/4, and to find 1/4 you divide by 4, so 50 ÷ 4 = 12.5 miles. Then you can add this to 100 and get your answer as 112.5 miles.
If you have a calculator, or if you're allowed a calculator, then a simpler way is to just do 2.25 × 50 = 112.5
Hope this helps!