Question 858788
 Starting at home, Christopher traveled uphill to the hardware store for 24 minutes at just 15 mph.
 He then traveled back home along the same path downhill at a speed of 30 mph.
What is his average speed for the entire trip from home to the hardware store and back?
:
Let d = the one-way distance to the store
:
let a = his average speed for the round trip
:
Write a time equation, time = dist/speed
time up + time back = total time
{{{d/15}}} + {{{d/30}}} = {{{(2d)/a}}}
multiply by 30a to clear the denominators, results:
2ad + ad = 30(2d)
3ad = 60d
divide both sides by d
3a = 60
a = 60/3
a = 20 mph is his average speed for the round trip