SOLUTION: Hi Guan heng left P and drove at 72kmph for 45 min. Then he increased his speed to 84kmph and drove for a further 40mins prior to reaching his home. His sister left P 20min earlie

Algebra ->  Customizable Word Problem Solvers  -> Misc -> SOLUTION: Hi Guan heng left P and drove at 72kmph for 45 min. Then he increased his speed to 84kmph and drove for a further 40mins prior to reaching his home. His sister left P 20min earlie      Log On

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Question 1170496: Hi
Guan heng left P and drove at 72kmph for 45 min. Then he increased his speed to 84kmph and drove for a further 40mins prior to reaching his home. His sister left P 20min earlier than him but she reached her home 25 min later.
What was his sisters average speed

Answer by Theo(13342) About Me  (Show Source):
You can put this solution on YOUR website!
rate * time = distance

72 kmph * 45/60 hours = 54 kilometers.
84 kmph * 20/60 hours = 56 kilometers.

total distance is 110 kilometers.
total time is (45 + 40) / 60 = 85 / 60 = 1.4166666667 hours.
average speed is 110 kilometers / 1.4166666667 = 77.64705882 kmph.

she left 20 minutes earlier and arrived 25 minutes later.
her time for the trip was (20 + 45 + 40 + 25) = 130 minutes / 60 = 2.166666667 hours.

her average speed was 110 / 2.166666667 = 50.76923077 kmph.

you could do this with fractions and should get the same answer.

doing it with fractions, you get:

72 kmph for 45/60 hours = a distance of 54 kilometers.
84 kmph for 40/60 hours = a distance of 56 kilometers.

total distance is 110 kilometers.
total time is 85/60 hours.

she started 20 minutes earlier and arrived
25 minutes later.
it took her a total time of 85 + 45 = 130/60 hours.

her average speed was 110 / (130/60) = 110 * 60 / 130 = 6600/130 kmph.

6600/130 = 50.76923077 kmph.
it's the same answer done with fractions for the intermediate calculations.