Explanation:
18.75 miles in 1/4 hour.
that is the ratio 18.75 / 1/4
but we want the miles in an hour.
so, how many 1/4 are in a whole ?
well, 4.
so we need to multiply the ratio by 4/4 (so that on one hand we multiply the denominator by 4 to make it a whole, and on the other hand we are not changing the value of the ratio) :
18.75 × 4 / (1/4 × 4) = 75 miles / 1 hour = 75 mph.