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