David travels one-third of the distance from LA to SF at a s

Question

David travels one-third of the distance from LA to SF at a speed of 40 miles an hour and the balance at a speed of 80 miles an hour. What is the average speed of David's travel from LA to SF?

A. 66.67 miles per hour

B. 48 miles per hour

C. 64 miles per hour

D. 60 miles per hour

E. 53.33 miles per hour

Bunuel · Answer

(Average speed) = (total distance)/(total time). Say the distance from LA to SF is d miles, then (Average speed) = (total distance)/(total time) = \frac{d}{\frac{(\frac{d}{3})}{40}+\frac{(\frac{2d}{3})}{80}}=\frac{d}{\frac{d}{120}+\frac{d}{120}}=60 miles per hour. Answer: D. P.S. Please name the topics properly. Check rule 3 here: rules-for-posting-please-read-this-before-posting-133935.html Also, please tag the topics properly: this is a distance/rate problem, not arithmetic. Thank you.

PareshGmat · Answer

Lets say he travels total 3 Miles........ (1)

So, speed for 1 mile = 40
Speed for next 2 miles = 80

Total time taken  =\frac{1}{40} + \frac{2}{80} = \frac{2}{40} ........ (2)

Average = \frac{Total distance}{Total time}

= 3 * \frac{40}{2}

= 60

Answer = D

GMATinsight · Answer

Another way to solve the question is by assuming the distance which will avoid the unwanted calculation of fractions

Let's assume the distance which is divisible by 3 as 1/3 Distance is Travelled @40 mph 
also Let's assume a distance that is divisible by 40 and 80 as well as TIME = DISTANCE/SPEED and Speeds are 40 and 80 mph

Let, Distance = 240 Miles

1/3 Distance = (1/3)*240 = 80 miles is Travelled @40 mph

i.e. TIME TAKEN = Distance / Speed = 80/40 = 2 hours

2/3 Distance = (2/3)*240 = 160 miles is Travelled @80 mph

i.e. TIME TAKEN = Distance / Speed = 160/80 = 2 hours

TOTAL DISTANCE = 240
TOTAL TIME = 2+2 = 4 Hours

AVERAGE SPEED = TOTAL DISTANCE / TOTAL TIME = 240/4 = 60 mph

Answer: option  D

ScottTargetTestPrep · Answer

We can use the average speed equation:

average speed = total distance/total time

We can also let the total distance be 600 miles. Therefore, we have:
 
average speed = 600/(200/40 + 400/80)

average speed = 600/(5 + 5)

average speed = 600/10

average speed = 60 mph

(Note: We can choose any number for the total distance between the two cities. In this case, 600 is a convenient number, making the resulting arithmetic easy to perform without a calculator.)

Answer: D

bumpbot · Answer

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David travels one-third of the distance from LA to SF at a s

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