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Circular gears

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by GmatKiss » Sat Aug 13, 2011 9:12 am
Circular gears P and Q start roating at the same time at constant speeds.gear P makes 10 revolutions per minute and gear Q makes 40 revolutions per minute.
How many seconds after the gears start roating will gear Q have made exactly 6 more revolutions then gear p?

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by Tani » Sat Aug 13, 2011 5:22 pm
Circle P takes 6 seconds to make one revolution and circle Q takes 1.5 seconds to make one revolution.

Let's work with the values we are given.
After 6 seconds, P will have made one and q will have made 4 - difference = 3.
We are looking for a difference of six so let's see what happens when we double that.

After 12 seconds P has made 2 revolutions (12/6 = 2)and Q has made 8 revolutions (12/1.5 = 8). Difference 6 = answer D
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by gmatboost » Mon Aug 15, 2011 9:30 am
It's quicker in my opinion to think about the difference as a rate and work from there.

If one makes 40 revs perm minute and the other makes 10, the faster one is gaining at 40-10 = 30 revs per minute.

To make 6 more revs will require 6/30 of a minute, or 1/5 or a minute, or 12 seconds.
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