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# The speed of a subway train is represented by the equation..

tagged by: AAPL

### Top Member

#### The speed of a subway train is represented by the equation..

The speed of a subway train is represented by the equation z=s^2+2s for all situations where 0 ≤ s ≤ 7, where z is the rate of speed in kilometers per hour and s is the time in seconds from the moment the train starts moving. In kilometers per hour, how much faster is the subway train moving after 7 seconds than it was moving after 3 seconds?

A. 4
B. 9
C. 15
D. 48
E. 63

The OA is D.

I don't have clear this PS question.

I just need to evaluate the speed equation for z(7) and z(3) and then find the difference between them? I appreciate if any expert explains it to me. Thank you so much.

### GMAT/MBA Expert

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AAPL wrote:
The speed of a subway train is represented by the equation z=s^2+2s for all situations where 0 ≤ s ≤ 7, where z is the rate of speed in kilometers per hour and s is the time in seconds from the moment the train starts moving. In kilometers per hour, how much faster is the subway train moving after 7 seconds than it was moving after 3 seconds?

A. 4
B. 9
C. 15
D. 48
E. 63

The OA is D.

I don't have clear this PS question.

I just need to evaluate the speed equation for z(7) and z(3) and then find the difference between them? I appreciate if any expert explains it to me. Thank you so much.
You're exactly right.

Her speed at 7 seconds: 7^2 + 2*7 = 49 + 14 = . 63
Her speed at 3 seconds: 3^2 + 2*3 = 9 + 6 = 15
The difference 63-16 = 48. The answer is D

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