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Marla starts running around a circular track at the same tim [#permalink]
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Carcass wrote:
Marla starts running around a circular track at the same time Nick starts walking around the same circular track. Marla completes 32 laps around the track per hour and Nick completes 12 laps around the track per hour. How many minutes after Marla and Nick begin moving will Marla have completed 4 more laps around the track than Nick?

(A) 5
(B) 8
(C) 12
(D) 15
(E) 20


Let t = the time (in HOURS) that it takes Marla to complete 4 more laps than Nick.
So, after t hours, we can write: (Marla's lap count) = (Nick's lap count) + 4

Now that we have a "word equation" we need only fill in the missing information

Marla completes 32 laps per hour
We can think of 1 lap as being a unit of distance.
So, 32 laps per hour is Marla's speed.

Distance = (speed)(time)
So, after t hours, Marla's lap count = 32t


Nick completes 12 laps around the track per hour
So, after t hours, Nick's lap count = 12t

We can now plug the above values into the word equation.
We get: 32t = 12t + 4
Subtract 12t from both sides to get: 20t = 4
Solve: t = 4/20 = 1/5 HOURS

1/5 hours = 12 minutes.

Answer: C

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Re: Marla starts running around a circular track at the same tim [#permalink]
We can also use the concept of Relative Speeds

Since they both are moving in the same direction, Their relative speed would be the positive difference between their speeds = 32 - 12 = 20 laps/hour

The distance here can be assumed as 4 laps

Therefore, T = 4/20 hours = 1/5 x 60 minutes = 12 minutes

Hence, option C
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Re: Marla starts running around a circular track at the same tim [#permalink]
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