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The price of a phone call consists of a standard connection

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The price of a phone call consists of a standard connection

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Source: Manhattan Prep

The price of a phone call consists of a standard connection fee, which is constant, plus a per minute charge. A 10-minute call costs $2.90 and a 16-minute call costs $4.40. How much does a 13-minute call cost?

A. $3.55
B. $3.57
C. $3.58
D. $3.65
E. $3.77

The OA is D

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Hi All,

We're told that the price of a phone call consists of a standard connection fee, which is constant, plus a per minute charge, a 10-minute call costs $2.90 and a 16-minute call costs $4.40. We're asked for the cost of a 13-minute call. This question can be solved in a couple of different ways - and there's a great 'rate shortcut' that you can use to avoid some of the extra math that comes with certain approaches.

Since there's a standard connection fee, we know that the difference between the costs of two calls is solely due to the number of minutes in the call. With the given information (about a 10-minute call and a 16-minute call), the difference in price comes down to the 6 minute difference in the length of the calls. Thus, the extra 6 minutes cost an extra $4.40 - $2.90 = $1.50. We're asked for the cost of a 13-minute call - which is exactly 'halfway' between those two prices. Half of $1.50 is $0.75, so we can add that to the cost of a 10-minute call to find the cost of the 13-minute call. $2.90 + $0.75 = $3.65

Final Answer: D

GMAT assassins aren't born, they're made,
Rich

_________________
Contact Rich at Rich.C@empowergmat.com

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BTGmoderatorLU wrote:
Source: Manhattan Prep

The price of a phone call consists of a standard connection fee, which is constant, plus a per minute charge. A 10-minute call costs $2.90 and a 16-minute call costs $4.40. How much does a 13-minute call cost?

A. $3.55
B. $3.57
C. $3.58
D. $3.65
E. $3.77
Letting f = the standard connection fee and n = the per-minute charge, we can create two equations, one for the 10-minute call and one for the 16-minute call::

f + 10n = 2.90

and

f + 16n = 4.40

Subtracting the first equation from the second, we have:

6n = 1.50

n = 0.25

Substituting 0.25 for n into the first equation, we see that f is:

f + 2.5 = 2.90

f = 0.4

So a 13-minute call costs 0.4 + 13 x 0.25 = $3.65.

Alternate solution:

If we let m = the per-minute charge, x = the number of minutes, b = the standard connection fee, and y = the total cost of an x-minute call, we will have y = mx + b. As we can see, this is a line, and the ordered pairs (10, 2.90) and (16, 4.40) are on this line. We are looking for the y-value when x = 13. We can observe that 13 is exactly halfway between 10 and 16; therefore, the y-value should be exactly halfway between 2.90 and 4.40 also. Thus y = (2.90 + 4.40)/2 = 7.30/2 = 3.65.

Answer: D

_________________
Scott Woodbury-Stewart Founder and CEO

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BTGmoderatorLU wrote:
Source: Manhattan Prep

The price of a phone call consists of a standard connection fee, which is constant, plus a per minute charge. A 10-minute call costs $2.90 and a 16-minute call costs $4.40. How much does a 13-minute call cost?

A. $3.55
B. $3.57
C. $3.58
D. $3.65
E. $3.77
$$? = f + 13c\,\,\,\,\left[ \$ \right]$$
The constant fee (f) and the minute-charge (c) will be considered in CENTS. (Amounts in cents are always integers!)
$$\left\{ \matrix{
\,f + 10c = 290 \hfill \cr
\,f + 16c = 440 \hfill \cr} \right.\,\,\,\,\,\,\,\,\mathop \Rightarrow \limits^{\left( + \right)} \,\,\,\,\,\,2f + 26c = 290 + 440\,\,\,\,\,\,\mathop \Rightarrow \limits^{{\rm{focus}}\,!} \,\,\,\,\,\,? = {{2f + 26c} \over 2} = 145 + 220 = 365\,\,\,\,\,\,\left[ {\,{\rm{cents}}\,} \right]$$

This solution follows the notations and rationale taught in the GMATH method.

Regards,
Fabio.

_________________
Fabio Skilnik :: GMATH method creator ( Math for the GMAT)
English-speakers :: https://www.gmath.net
Portuguese-speakers :: https://www.gmath.com.br



Last edited by fskilnik@GMATH on Wed Oct 17, 2018 8:02 am; edited 1 time in total

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BTGmoderatorLU wrote:
Source: Manhattan Prep

The price of a phone call consists of a standard connection fee, which is constant, plus a per minute charge. A 10-minute call costs $2.90 and a 16-minute call costs $4.40. How much does a 13-minute call cost?

A. $3.55
B. $3.57
C. $3.58
D. $3.65
E. $3.77

The OA is D
A 10-minute call costs $2.90 and a 16-minute call costs $4.40
$4.40 - $2.90 = $1.50, and 16 minutes - 10 minutes = 6 minutes

So, the extra $1.50 paid for extra 6 minutes of calling
So, $0.75 would pay for 3 minutes of calling

How much does a 13-minute call cost?
A 10-minute call costs $2.90
So, the cost of a 13-minute call = $2.90 + $0.75 = $3.65

Answer: D

Cheers,
Brent

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Brent Hanneson – Creator of GMATPrepNow.com
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