SOLUTION: 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

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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?


Answer by ankor@dixie-net.com(22740) About Me  (Show Source):
You can put this solution on YOUR website!
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%2F30 + %28.5d%29%2F30 + %28.5d%29%2F70 = %282d%29%2Fa
Add like terms
%281.5d%29%2F30 + %28.5d%29%2F70 = %282d%29%2Fa
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