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# help

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Jimmy travels from Point A to Point B at a speed of 40 mph, then using the same road, Jimmy travels from Point B to Point A at a speed of 60 mph.

What is the average speed for the round trip?

Why is the answer not 50?

Jun 25, 2020

#1
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40+60=100

100/2=50

should be 50 miles per hour, but remember, you have to also calculate the leftovers!

Jun 25, 2020
edited by whymenotsmart  Jun 25, 2020
#2
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Let the distance from A to B be L miles

time from A to B:  t1 = L/40 hours

time from B to A:  t2 = L/60 hours

Average speed for round trip = total distance/total time

so average speed = 2L/(t1 + t2)  = 2L/(L/40 + L/60) = 2/(1/40 + 1/60)  = 48 mph

Jun 25, 2020
#3
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You have to take into account the TIME at each speed.....the TIME to travel the same distance will be LONGER for 40 m/hr vs 60 m/hr so the AVERAGE will be weighted more toward the 40 m/hr ...

40 x t1 = d    d/40 = t1

60 x t2 = d    d/60 = t2

2d/avg = t1 + t2 = d/40 + d/60

2d/avg = (3d+2d)/120

2/avg = 5/120

avg/2 = 120/5

avg = 240/5 = 48 m/hr

Jun 25, 2020
edited by ElectricPavlov  Jun 25, 2020