Question 772723
A student drives from home to school at an average speed of 40 km/hr and returns home at an average speed of 60 km/hr.
 Find the average speed for the entire trip? 
:
Let d = the one-way distance
Let a = the average speed for the round trip
:
Write time equation; time = dist/speed
:
To sch time + return time = total time
{{{d/40}}} + {{{d/60}}} = {{{(2d)/a}}}
multiply equation by 120 a to clear the denominators, results:
3ad + 2ad = 120(2d)
5ad = 240d
divide both sides by d
5a = 240
a = 240/5
a = 48 mph is the average speed
:
Why is your answer not 50 km/hr? Because you spend more time driving 40 
than you do a 60.