What will the ratio of birds to fish in a certain pet store be after t

Question

What will the ratio of birds to fish in a certain pet store be after the store buys seven additional fish?

(1) Before the purchase, the pet store has birds and fish, respectively, in a 6 to 7 ratio.
(2) The resultant ratio of birds to fish in the pet store would be the same if, instead of buying seven fish, the pet store sold four birds.

kolodits · Answer

We need the ratio: b/(f+7).
1. We have b/f = 6x/7x. Since we need 6x/(7x+7) and we don't know what is x, this is clearly insufficient.
2. According to this statement: b/(f+7)=(b-4)/f. After some manipulation somehow I got: b/(f+7)= 4/7. So apperantly this is sufficient. I'm really not sure it's correct.
Anyway, the answer is B.

Sent from my Redmi 4 using  GMAT Club Forum mobile app

mohshu · Answer

can u plz explain the manipulation..

goforgmat · Answer

how did you get the ratio b/(f+7)= 4/7 ?
Can you explain?

kolodits · Answer

Sorry about that. It's a little bit difficult to show all the maths using the cell app.
According to statement 2:  \frac{b}{(f+7)}  =  \frac{(b-4)}{f} 
multiply by the denominator: b*f = (b-4)*(f+7) 
 b*f = b*f +7b - 4f -28 
 0 = 7b - 4f -28 
 7b = 4f + 28 
 \frac{7b}{(4f + 28)} = 1 
Finally we have to divide by 7 and multiply by 4 to get: \frac{b}{(f+7)} = \frac{4}{7} 
This is what we were looking for.
If we want to verify this (actually no match time for this during the exam) we can choose values which satisfy this ratio. 
let's say b = 8 , f = 7 -->  \frac{b}{(f+7)} = \frac{8}{(7+7)}= \frac{8}{14} = \frac{4}{7} 
In addition: \frac{(b-4)}{f} = \frac{(8-4)}{7} = \frac{4}{7} 
This works for values: b = 16, f = 21, since f+7 = 28, b-4 = 12, which fulfill the ratio we have found.

Hope this is clear.

streetking · Answer

Statement (2) provides 
bf+7=b−4f
.

If we're clever, we can go straight to the answer without having to slog through this algebra. Notice that the difference between these two "after" ratios is four birds and seven fish. Furthermore, we are told that these two "after" ratios are equal. But adding to a ratio can only leave the ratio unchanged if what we added is the same ratio as what we added to. In other words, since we know that adding four birds and seven fish does not change the "after" ratio, the "after" ratio must itself be a 
4
 to 
7
 ratio of birds to fish. Statement (2) is sufficient, and the answer is B.

bumpbot · Answer

Automated notice from GMAT Club BumpBot: 

A member just gave Kudos to this thread, showing it’s still useful. I’ve bumped it to the top so more people can benefit. Feel free to add your own questions or solutions.

 This post was generated automatically.

What will the ratio of birds to fish in a certain pet store be after t

Prep Toolkit

Top 5 GMAT Debrief Videos

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards

GMAT Data Sufficiency (DS) Questions

What will the ratio of birds to fish in a certain pet store be after t

Prep Toolkit

615 to 715 in 15 Days (Ria's Strategies)

More than Quant/Verbal Abilities: Speed vs. Accuracy

745 with Only Self-Prep and GMAT Club

From 555 to 765: 210-point Improvement

Perfect 805 Score Debrief by Julia

My Rewards