RPM question

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capnx: Master | Next Rank: 500 Posts; Posts: 268; Joined: Sat Aug 15, 2009 9:22 amFollowed by: 1 member; Thanked: 19 times; GMAT Score:700

by capnx » Tue Oct 13, 2009 7:23 pm

A circular rim 28inches in diameter rotates the same number of inches per second as a circular rim 35inches in diameter. If the smaller rim makes X revolutions per second, how many revolutions per minute does the larger rim make in terms of x?

48pi/x
75x
48x
24x
x/75

could someone please explain the formulas related to revolutions (rpm and rps)? is rpm = circumference/min?
thanks

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Source: Beat The GMAT — Problem Solving | Tue Oct 13, 2009 7:23 pm

2010gmat: Master | Next Rank: 500 Posts; Posts: 371; Joined: Fri Mar 06, 2009 2:48 am; Thanked: 27 times; GMAT Score:740

by 2010gmat » Tue Oct 13, 2009 7:56 pm

X/x = 35/28

--> x = (4/5)*X/second

--> x = (4/5)*X*60/minute = 48X

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