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100 points for $49 worth of Veritas practice GMATs FREE VERITAS PRACTICE GMAT EXAMS Earn 10 Points Per Post Earn 10 Points Per Thanks Earn 10 Points Per Upvote The speed of a subway train is represented by the equation.. tagged by: AAPL This topic has 2 expert replies and 0 member replies 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 â‰¤ s â‰¤ 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 Legendary Member Joined 14 Jan 2015 Posted: 2667 messages Followed by: 122 members Upvotes: 1153 GMAT Score: 770 AAPL wrote: The speed of a subway train is represented by the equation z=s^2+2s for all situations where 0 â‰¤ s â‰¤ 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 _________________ Veritas Prep | GMAT Instructor Veritas Prep Reviews Save$100 off any live Veritas Prep GMAT Course

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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 â‰¤ s â‰¤ 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

.
z(7) = 7^2 + 2(7) = 49 + 14 = 63

z(3) = 3^2 + 2(3) = 9 + 6 = 15

So the difference is 63 - 15 = 48.

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