Time/Distance

[grkmurthy]

John drives from Los Angels to San Diego at an average speed of 60 miles per hour and returns at an average speed of 70 miles per hour. What is John's average speed (in nearest miles per hour) for the entire trip assuming that Los angels and San Diego are 100 miles apart.

(1) 67.5
(2) 64.6
(3) 65.0
(4) 66.5
(5) 68.6

[ksvickram]

If same distance is covered in two trips at different speeds, the average speed of the entire journey is just the harmoni mean of the tow speeds.

Thus, (2*x*y) / (x+y) should give the result and the answer is 64.6 approx.