Data Sufficiency

Ette004 wrote:Hey guys,

I'm just analyzing my mistakes in the GMAT Prep test and I can't figure out the solution for the following question:

In a Pet-Shop 1/3 of the pets are dogs and 1/5 of the pets are birds. How many of the dogs are pets?
(1) There are 30 birds in the Pet-Shop
(2) There are 20 more Birds than dogs in the Pet-Shop

The correct answer is D - Both statements seperately are sufficient.
(1) is clear to me - but I'm struggling to see how (2) is sufficient to identify the number of dogs.

Do you guys have an idea?

Cheers
Thomas

The correct question is:

At a certain pet shop, 1/3 of the pets are dogs and 1/5 of the pets are birds. How many of the pets are dogs?

(1) There are 30 birds at the pet shop.
(2) There are 20 more dogs than birds at the pet shop.

Let the total number of pets = P, birds = B and dogs = D.
Then from the question we have,
B = P/5
D = P/3

(1) No. of birds = 30
P/5 = 30, so we can find P and hence we can find the no. of dogs; SUFFICIENT.

(2) There are 20 more dogs than birds implies B + 20 = D
So, P/5 + 20 = P/3
Here again we can find P and hence we can find number of dogs; SUFFICIENT.

The correct answer is D.