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# M21-12

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Math Expert
Joined: 02 Sep 2009
Posts: 59032

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16 Sep 2014, 01:10
00:00

Difficulty:

15% (low)

Question Stats:

81% (01:37) correct 19% (01:42) wrong based on 148 sessions

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If the diameter of each of a car's wheels is 0.4 meters, approximately, what was the average speed of the car for its three-hour trip in which each wheel made 100,000 revolutions?

A. 33 kmh
B. 35 kmh
C. 42 kmh
D. 47 kmh
E. 51 kmh

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Math Expert
Joined: 02 Sep 2009
Posts: 59032

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16 Sep 2014, 01:10
1
Official Solution:

If the diameter of each of a car's wheels is 0.4 meters, approximately, what was the average speed of the car for its three-hour trip in which each wheel made 100,000 revolutions?

A. 33 kmh
B. 35 kmh
C. 42 kmh
D. 47 kmh
E. 51 kmh

The circumference of the wheel is $$0.4\pi$$ or approximately 1.2 meters. The car went $$100,000*1.2$$ meters or $$100*1.2 = 120$$ km. Because we used 3 instead of 3.14, the actual average speed of the car must be somewhat higher than $$\frac{120}{3} = 40$$kmh. The best answer is C.

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Intern
Joined: 28 May 2017
Posts: 7

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16 Oct 2018, 19:07
I feel this solution is wrong.

Total distance without rounding off = 40*3.414 km
Avg Speed = 40*3.414/3 = 45.52km/hr
None of the options has the answer.
In this case, Option D is the closest possible choice.
Math Expert
Joined: 02 Sep 2009
Posts: 59032

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16 Oct 2018, 20:53
sarvesh93sah wrote:
I feel this solution is wrong.

Total distance without rounding off = 40*3.414 km
Avg Speed = 40*3.414/3 = 45.52km/hr
None of the options has the answer.
In this case, Option D is the closest possible choice.

__________________________
$$\pi = 3.141...$$, not 3.414.
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Intern
Joined: 28 May 2017
Posts: 7

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16 Oct 2018, 23:33
Bunuel wrote:
sarvesh93sah wrote:
I feel this solution is wrong.

Total distance without rounding off = 40*3.414 km
Avg Speed = 40*3.414/3 = 45.52km/hr
None of the options has the answer.
In this case, Option D is the closest possible choice.

__________________________
$$\pi = 3.141...$$, not 3.414.

Re: M21-12   [#permalink] 16 Oct 2018, 23:33
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# M21-12

Moderators: chetan2u, Bunuel