Monitors manufactured at a certain plant in November

[melguy]

Of the computer monitors manufactured at a certain plant in November, 5/7 were approved for sale by quality control. Of those monitors approved for sale, 7/8 were sold in stores, and of the monitors sold in stores, 1/3 were not sold at the suggested retail price. If 3500 of the monitors manufactured in November were sold at the suggested retail price, how many monitors were manufactured at this plant in November?

(A) 4,200
(B) 6,400
(C) 7,000
(D) 8,400
(E) 9,000

I have successfully solved this problem but in the CAT I was out of time so picked the wrong answer. I am looking for a quick approach to this problem.

Fractions are 5/7, 7/8 and 1/3 (not sold) or 2/3 sold. Considering the info the answer must be divisible by 3,7 and 8. We have only one answer which meets the criteria. Please confirm this approach is correct. Thanks

For those of you attempting the Q - OA is D

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Hi melguy,

I like your idea and it WILL help you to eliminate some of the answers; just not all of them.

You are correct that the correct answer must be divisible by 3, 7 and 8. However, you actually have TWO answers that fit: A and D. One of them is correct, one of them is not, so you could take an educated guess.

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

[ganeshrkamath]

Of the computer monitors manufactured at a certain plant in November, 5/7 were approved for sale by quality control. Of those monitors approved for sale, 7/8 were sold in stores, and of the monitors sold in stores, 1/3 were not sold at the suggested retail price. If 3500 of the monitors manufactured in November were sold at the suggested retail price, how many monitors were manufactured at this plant in November?

(A) 4,200
(B) 6,400
(C) 7,000
(D) 8,400
(E) 9,000

I have successfully solved this problem but in the CAT I was out of time so picked the wrong answer. I am looking for a quick approach to this problem.

Fractions are 5/7, 7/8 and 1/3 (not sold) or 2/3 sold. Considering the info the answer must be divisible by 3,7 and 8. We have only one answer which meets the criteria. Please confirm this approach is correct. Thanks

For those of you attempting the Q - OA is D

Let the total number of monitors manufactured be T.

Number of monitors manufactured that were sold at the suggested retail price
= 5/7 * 7/8 * (1 - 1/3) * T = 3500
5/7 * 7/8 * 2/3 * T = 3500
T = 3500 * 8 * 3 / 10
T = 350 * 24
T = 8400

Choose D

The method that you suggested depends on the answer options.
There might be two options that satisfy these criteria in which case you would've wasted valuable time.

Cheers

[melguy]

Thanks all for your input.

Rich - I guess I will stick to my old strategy of plugging in numbers. I normally start plugging in numbers from the bottom.

May be not a very good idea for me to change a strategy (that works well for me) last min

[GMATGuruNY]

Of the computer monitors manufactured at a certain plant in November, 5/7 were approved for sale by quality control. Of those monitors approved for sale, 7/8 were sold in stores, and of the monitors sold in stores, 1/3 were not sold at the suggested retail price. If 3500 of the monitors manufactured in November were sold at the suggested retail price, how many monitors were manufactured at this plant in November?

(A) 4,200
(B) 6,400
(C) 7,000
(D) 8,400
(E) 9,000

We can plug in the answers, which represent the total number of monitors manufactured.
As melguy noted, the correct answer choice must be a multiple of the three denominators in the problem: 7, 8, and 3.
Only A and D are viable.
Since 1/3 of the monitors sold in stores were NOT sold at the suggested retail price, the 3500 monitors that WERE sold at the suggested retail price represent 2/3 of the monitors sold in stores.
Since (2/3)4200 = 2800, answer choice A is too small.

The correct answer is D.