An airplane departs from LA and flies to NY every 30 minutes

An airplane departs from LA and flies to NY every 30 minutes. The trip takes 5 hours and 5 minutes. An airplane takes off from NY at the same time that one takes of from LA and flies to LA at the same speed. How many pianos does it pass going in the opposite direction?

Solution

The trip takes 5 hours and 5 mins, or 305 minutes.

A flight takes off every 30 mins = 305/30 = ~10 flights, plus the one that took off at the same time, so 11 flights take off during that time.

Another 10 flights were already en route when the plane took off, so the answer is 11 + 10 = 21.

17

To rephrase it and put everything together, 8 flights took off before the plane, one took off at the same time, and 8 took off when it was en route to LA, 8 + 1 + 8 = 17