Safal wrote:

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

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

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