I have encountered the following question in the famous "5LB book of GRE" book
If the zoo currently has 80 total birds, what is the samllest number of birds that could be added such that atleast 20% of the animals at the zoo would be birds?
(A) 10
(B) 80
(C) 100
(D) 125
(E) 200
The correct answer is 100 and in the answer sheet it stats "If the zoo has 80 birds, which make up to 10% of the total number, then there are 800 total animal"
Can this be derived from the question or is the question incomplete?
Yep - definitely incomplete.
If you don't know either the total number of animals or some ration of birds to animals, there is no way the question can be answered.
My guess is that the test preparer left out a phrase and the proof reader goofed.