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If electricity costs x cents per kilowatt-hour for the first 30 kilowa

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Bunuel
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If electricity costs x cents per kilowatt-hour for the first 30 kilowa [#permalink] New post   29 Dec 2014, 08:22
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Tough and Tricky questions: Word Problems.



If electricity costs x cents per kilowatt-hour for the first 30 kilowatt-hours, and y cents per kilowatt-hour for each additional kilowatt-hour, what is the total cost of z kilowatt-hours, where z > 30?

A. 30x + (z – 30)y
B. 30y – 30x + yz
C. 30(x – y + z)
D. (z – 30)x + 30y
E. 30x + (y – 30)z

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Re: If electricity costs x cents per kilowatt-hour for the first 30 kilowa [#permalink] New post   29 Dec 2014, 08:40
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The first 30 kWh cost x cents each, so in total they cost 30x ct. z needs to be decreased by 30 because we have already accounted for the first 30kWh, and then multiplied with y. So we get \(30x+(z-30)*y\) . Answer A.
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Re: If electricity costs x cents per kilowatt-hour for the first 30 kilowa [#permalink] New post   30 Dec 2014, 11:32
Bunuel wrote:

Tough and Tricky questions: Word Problems.



If electricity costs x cents per kilowatt-hour for the first 30 kilowatt-hours, and y cents per kilowatt-hour for each additional kilowatt-hour, what is the total cost of z kilowatt-hours, where z > 30?

A. 30x + (z – 30)y
B. 30y – 30x + yz
C. 30(x – y + z)
D. (z – 30)x + 30y
E. 30x + (y – 30)z

Kudos for a correct solution.


Cost of electricity for the first 30 kw-hours = 30x
Cost of electricity for the remaining kw-hours = (z-30)y

Combining both = 30x+(z-30)y

A
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Re: If electricity costs x cents per kilowatt-hour for the first 30 kilowa [#permalink] New post   30 Dec 2014, 19:19
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Answer = A. 30x + (z – 30)y

First 30 units, cost = 30*x = 30x

Remaining (z-30) units, cost = (z-30)*y

Total = 30x + (z – 30)y
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Re: If electricity costs x cents per kilowatt-hour for the first 30 kilowa [#permalink] New post   08 Jan 2015, 08:03
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Bunuel wrote:

Tough and Tricky questions: Word Problems.



If electricity costs x cents per kilowatt-hour for the first 30 kilowatt-hours, and y cents per kilowatt-hour for each additional kilowatt-hour, what is the total cost of z kilowatt-hours, where z > 30?

A. 30x + (z – 30)y
B. 30y – 30x + yz
C. 30(x – y + z)
D. (z – 30)x + 30y
E. 30x + (y – 30)z

Kudos for a correct solution.


OFFICIAL SOLUTION:

(A) From the given information, you can establish that it costs 30x for the first 30 kilowatt-hours.

Since we are trying to determine the total cost of z kilowatt-hours, but we have already determined the cost of the first 30 kilowatt-hours, we just have to determine the cost of the remaining z – 30 kilowatt-hours at a rate of y cents per kilowatt-hour. This cost is:(z – 30)y.Therefore, the total cost is:30x + (z – 30)y.

The correct answer is choice (A).
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Re: If electricity costs x cents per kilowatt-hour for the first 30 kilowa [#permalink]
08 Jan 2015, 08:03
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