What speed covers 15miles in 50 minutes?
15miles in 50minutes = 3miles in 10mins (divide by 5)
3miles in 10minutes = 18miles in 60mins (multiply by 6)
so 18mph
If you are struggling then practise these lots, you will have to do a lot of them very quickly and all in your head (no calculators) if you are to pass stage 1... and the above example is a fairly simply one.