Andrew drove from A to B at 60 miles per hour. Then he realized that h

Question

Andrew drove from A to B at 60 miles per hour. Then he realized that he forgot something at A, and drove back at 80 miles per hour. He then zipped back to B at 90 mph. What was his approximate average speed in miles per hour for the entire night?

Culled from M-GMAT word translation guide.
Is there any concise way of solving this problem?

Cos the solution given by M-gmat wasn't a CAT-wise solution, IMO. Twas quite long considering the nature of the question.
I'll appreciate with kudos any solution that didn't draw up a big table.

Official answer was given as 74.5. No options were given.

ENGRTOMBA2018 · Answer

All average speed questions are dealt in the same way.

Average Speed = \frac {Total distance} { Total Time taken}

Total distance = A to B + B to A+ A to B = S+S+S=3S

Total time = S/60 + S/80+S/90

Thus,  Average speed = \frac{3S}{S/60 + S/80+S/90} , after cancelling out the S from the Numerator and the Denominator, you are left with,

Average speed = \frac{30}{29/72} = \frac {30*72}{29} , now realize that 30/29 is just >1 and thus the average speed = just more than 72 mph.

Actual value :  Average speed = \frac{30}{29/72} = 74.48 mph

Hope this helps.

P.s.: IMO, you dont need the fancy tables or methods if you know what you are dealing with.

warriorguy · Answer

Just picked LCM of 60,80 and 90 = 720
Calculated total time in each session = 29

Avg speed = 720*3/29= 74.48

Andrew drove from A to B at 60 miles per hour. Then he realized that h

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