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# The speed of a subway train is represented by the equation z=s2+2s

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Math Expert
Joined: 02 Sep 2009
Posts: 60727
The speed of a subway train is represented by the equation z=s2+2s  [#permalink]

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17 Feb 2016, 04:27
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Difficulty:

5% (low)

Question Stats:

91% (01:29) correct 9% (01:54) wrong based on 65 sessions

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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

Kudos for correct solution.

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Re: The speed of a subway train is represented by the equation z=s2+2s  [#permalink]

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18 Feb 2016, 04:40
Bunuel 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

Kudos for correct solution.

After 7 sec z=49+14=63 km/hr

After 3 sec z=9+6=15 km/hr

difference= 48 km/he

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Re: The speed of a subway train is represented by the equation z=s2+2s  [#permalink]

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23 Feb 2016, 03:23
Bunuel 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

Kudos for correct solution.

Given: z=s^2+2s for 0≤s≤7

z(3) = 3^2 + 2*3 = 15
z(7) = 7^2 + 2*7 = 63

Therefore z(7) - z(3) = 63 - 15 = 48 km/hr
Option D
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Re: The speed of a subway train is represented by the equation z=s2+2s  [#permalink]

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10 Jun 2019, 19:36
Bunuel 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

Kudos for correct solution.

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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Re: The speed of a subway train is represented by the equation z=s2+2s   [#permalink] 10 Jun 2019, 19:36
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