Question 791031
 a car travels from station a to b at 30kmph during its return trip to A,it travels at 30kmph for half of the distance and at 70kmph for next half of the distance.find average speed of the car?
:
Let d = the distance from a to b
then
.5d = half the distance from a to b
:
Let a = average speed for the round trip
:
Write a time equation: time = dist/speed.
:
{{{d/30}}} + {{{(.5d)/30}}} + {{{(.5d)/70}}} = {{{(2d)/a}}}
Add like terms
{{{(1.5d)/30}}} + {{{(.5d)/70}}} = {{{(2d)/a}}}
Multiply by 210a, resulting in
7a(1.5d) + 3a(.5d) = 210(2d)
10.5ad + 1.5ad = 420d
12ad = 420d
divide both sides by d
12a = 420
a = 420/12
a = 35 mph is the average speed
:
You can check this yourself, use d=60 to find the actual time at each speed