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A car traveled the first 1/3 of the distance from Town X to Town Y at an average speed of 20 miles per hour, it traveled the next 1/3 of the distance from Town X to Town Y at an average speed of 30 miles per hour, and it traveled the final 1/3 of the distance from Town X to Town Y at an average speed of 36 mile per hour. What was the car's average speed, in miles per hour, when it traveled from Town X to Town Y?

A) 27
B) 28
C) 28 2/3
D) 29
E) 30 1/2

Set the distance of each portion as X. Then the total distance is 3X. We need the total time from the 3 portions added up.

Note the answer is not (20 + 30 + 36) / 3 because the three portions don't share the same time used, but rather the same distance instead.

t = D / v so the three times are x/20, x/30 and x/36. We can use an easy common denominator 360 to speed up calculations.

\(1/20 + 1/30 + 1/36 = 18/360 + 12/360 + 10/360 = 40/360 = 1/9\).

Finally 3X / (X/9) = 27.

Ans: A
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Let us assume a suitable number (say 180 miles) for the length of each segment.
Total time: 180/20 + 180/30 + 180/36 = 20
Total distance: 180*3 = 540
Average speed: 540/20 = 27 mph.
ANS: A
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coreyander
A car traveled the first 1/3 of the distance from Town X to Town Y at an average speed of 20 miles per hour, it traveled the next 1/3 of the distance from Town X to Town Y at an average speed of 30 miles per hour, and it traveled the final 1/3 of the distance from Town X to Town Y at an average speed of 36 mile per hour. What was the car's average speed, in miles per hour, when it traveled from Town X to Town Y?

A) 27
B) 28
C) 28 2/3
D) 29
E) 30 1/2
Av Speed= Total distance/total time
Total distance=540
Total time=T1+T2+T3
T1=180/20
T2=180/30
T3=180/36
Av Speed=540/(9+6+5)
Av speed= 27
A:)
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lets consider total distance = 90 km
time = distance/speed
first 30 km speed is 20=30/20=3/2
next 30 km speed is 30=30/30=3/3
last 30 km speed is 36=30/36=5/6
total time = 3/2*3/3*5/6=20/6

average speed= total distance/ total time = 90/(20/6)= 9*3=27

ANS:A
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coreyander
A car traveled the first 1/3 of the distance from Town X to Town Y at an average speed of 20 miles per hour, it traveled the next 1/3 of the distance from Town X to Town Y at an average speed of 30 miles per hour, and it traveled the final 1/3 of the distance from Town X to Town Y at an average speed of 36 mile per hour. What was the car's average speed, in miles per hour, when it traveled from Town X to Town Y?

A) 27
B) 28
C) 28 2/3
D) 29
E) 30 1/2

The car is travelling at different speeds over equal distances. We know that when the car travels with different speeds a and b over two equal distances, the average speed is 2ab/(a+b).
But here we have 3 equal distances so let's simply use the concept Average Speed = Total Distance/Total Time.

Let's assume that each leg of distance (the 1/3rd distance) = 360 (the easiest number that came to mind that is divisible by 20, 30 and 36)

Total Distance = 360 * 3 miles
Total Time taken = 360/20 + 360/30 + 360/36 = 18 + 12 + 10 = 40 hrs

Average Speed = 360*3/40 = 27 mph

Answer (A)

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