Quantitative Review DS 58)

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El Cucu: Master | Next Rank: 500 Posts; Posts: 236; Joined: Fri Nov 28, 2008 9:16 am; Thanked: 9 times

by El Cucu » Wed Feb 18, 2009 9:20 am

Hi Guys need an explanation regarding the total distance and the radii of the wheel.

58. What is the number of 360 degree rotations that a bicycle wheel made while rolling 100 meters in a straight line without slipping?

1) The diameter of the bicycle wheel, including the tire, was 0,5 meter.

2) The wheel made twenty 360 degree rotations per minute.

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Source: Beat The GMAT — Data Sufficiency | Wed Feb 18, 2009 9:20 am

shulapa: Senior | Next Rank: 100 Posts; Posts: 35; Joined: Thu Nov 13, 2008 8:05 am; Thanked: 6 times

by shulapa » Wed Feb 18, 2009 9:28 am

Hi,

In order to know the number of rotation the wheel has done while passing through 100m we need to know the circumference of the wheel. The we can use this equation:
Distance/perimeter = number of rotation

(1) provides us with the wheel diameter so perimeter can be found via:

P=2pi*r=pi*d

Thus, sufficient.

(2) We are given the rotation speed of the wheel but we don't have any thing to connect speed to the distance (like time to pass the entire path).

Thus, insufficient

Therefore, A

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El Cucu: Master | Next Rank: 500 Posts; Posts: 236; Joined: Fri Nov 28, 2008 9:16 am; Thanked: 9 times

by El Cucu » Wed Feb 18, 2009 9:32 am

Still can't imagine why having the circumference we are able to answer the problem.

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tini: Junior | Next Rank: 30 Posts; Posts: 27; Joined: Sat Dec 29, 2007 1:37 amFollowed by: 1 member; Thanked: 2 times

by tini » Wed Feb 18, 2009 9:42 am

that's because when a wheel rolls once it covers a distance equal to the length of its circumference.

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