Question 946840
Starting at home, Christopher traveled uphill to the hardware store for 50 minutes at just 6 mph. 
He then traveled back home along the same path downhill at a speed of 12 mph.
What is his average speed for the entire trip from home to the hardware store and back?
:
let a = his average speed of the trip
let d = the one-way distance
:
Write a time equation; time = dist/speed
Uphill time + downhill time = total time
{{{d/6}}} + {{{d/12}}} = {{{(2d)/a}}}
Multiply equation by 12a
12a*{{{d/6}}} + 12*{{{d/12}}} = 12a*{{{(2d)/a}}}
Cancel the denominators
2ad + ad  = 24d
3ad = 24d
divide both sides by d
3a = 24
 a = 24/3
 a = 8 mph is the av speed of the round trip