A bicycle climbs a certain hill at 10 miles per hour and returns at 20 miles per hour. What is its average speed for the entire trip?
It's NOT 15 mph {as many people would think !!!}
There's an easy "formula" for this:
[2(R1)(R2)] / [ R1 + R2 ] where R1 and R2 are the rates ... so we have
[2(10)(20)] / [10 + 20] =
400 / 30 =
about 13.33 mph