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I plugged in the values.

by the the time Rubin completed 3 laps in 15 minutes , Warren completed only 15/7 ~2.1 of the lap

plug in 17.5 and we get 17.5/5 = 3.5 rounds for Rubin and 17.5/7 = 2.5 Rounds for for warren
The difference is thus 1 round so :E 17.5 is the answer
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Warren runs one lap in 7 mins
Rubin runs one lap in 5 mins

We can say in 7 mins Warren completes 1 lap while Rubin complete 1+2/5th of the second lap.
in the next 7 mins warren completes to laps while Rubin completes 1+1+ 2/5+2/5 (4/5 lap ahead of warren)

to complete 1/5th of lap Rubin will require 3.5 mins.

Total time required by Rubin to be ahead of Warren by exactly one lap = 7 + 7 + 3.5 = 17.5 mins
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Warren and Rubin race around an oval track. Warren runs at a uniform rate of one lap every 7 minutes, and Rubin runs at a uniform rate of one lap every 5 minutes. If they both start at the same time. How many minutes will it take for Rubin to get exactly one lap ahead of Warren.

(A) 12
(B) 13 1/2
(C) 15
(D) 16.7
(E) 17.5

Rate of Warren = \(\frac{1}{7}\) lap/minute

Say, distance travelled by Warren = d laps

So, time taken by Warren = \(\frac{d}{1/7}\) = 7d

Rate of Rubin = \(\frac{1}{5}\) lap/minute

Distance travelled by Rubin = d+1 laps

Time taken = \(\frac{(d + 1)}{(1/5)}\) = 5d + 5

Now we can equate their times as both ran for equal duration

7d = 5d + 5

d =\(\frac{5}{2}\)

Minutes will it take for Rubin to get exactly one lap ahead of Warren = 5d + 5

\(5 * \frac{5}{2} + 5\)

\(\frac{25}{2} + 5\)

17.5 (E)
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I tried plugging values and got it wrong because 15 was very close to the answer so I assumed D but it was wrong. Tried with relative speed and it is much faster.

W = 1/7 laps/minute, R = 1/5 laps a minute

Relative rate = 1/5-1/7 = 2/35 laps a minute

R gains 2/35 laps over W a minute, how much time to gain ONE full lap?

2/35 * T = 1

T = 35/2 = 17.5 minutes.
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