.
In driving to keep an appointment in another city. Harry estimated that he would arrive on time if he maintained a constant rate
of 40 mph. When he was 10 mile from his destination, however, he was forced to detour over a longer route and reduce his speed
to 30 mph. He arrived 15 minutes late. How long was the detour?
~~~~~~~~~~~~
It can be solved mentally by reasoning, without using equations
When Harry was 10 miles from his destination, the expected time to arrive was = of an hour, or 15 minutes.
But moving detour, Harry spent 15 minutes + 15 minutes = 30 minutes = 0.5 of an hour.
Hence, the deture was 40 mph x 0.5 of an hour = 20 miles long. ANSWER
Solved.