A research scientist wants to study a certain attribute?

Question

A research scientist wants to study a certain attribute of
dogs. It is estimated that approximately 5 percent of all
dogs have this attribute. If the scientist wants to study a
sample of N dogs having the attribute, approximately
how many dogs should be screened in order to obtain the
desired sample size?
(A) N/5
(B) 5N
(C) 2ON
(D) 105N.
(E) 120N

Answer 1

C

Here's how to do it:

5% = 0.05 = 1/20

So, out of every 20 dogs, 1 has it.

Thus, if you want N dogs with this attribute, you need 20N dogs. To get this, I just multiplied the fraction above by 1 = N/N:
1/20 * N/N = N/20N

See? I get N dogs with the attribute from a population of 20N dogs.

Answer 2

C--5% is 1/20
N = 1/20 of screening group
20N = screening group

Answer 3

C. 20N

Answer 4

I'm thinking "C".