I would solve this through plugging in numbers and then checking my answer choices.

Assume 1 newspaper A was sold and 4 newspaper Bs were sold

Therfore 1*1$$ + 4*1.25$ = 6$

Therefore A's revenue percentage ie; r = (1/6)*100 = 16,67%
A's copy percentage ie; p = (1/5)*100 = 20%

The answer choice, if you plug in the numbers you will find that r = 400p/(500-p) = 8000/480 = 16,67%

Regards
Anup

Last Sunday a certain store sold copies of Newspaper A for $1 and Newspaper B for 1.25. If r percent of the stores revenue came from newspaper sales of Paper A and if p percent of the newspapers sold that day were A, which of the following expresses r in terms of p?

100p/125-p
150p/250-p
300p/375-p
400p/500-p
500p/625-p

Plug in numbers that make the math easy.

If the number of copies of A = the number of copies of B, then p=50, since 1/2 of the newspapers sold are A.

If 4 copies of each type of newspaper are sold, then the total revenue will be an integer:
Revenue from 4 copies of A = 4*1 = 4.
Revenue from 4 copies of B = 4*1.25 = 5.
Total revenue = 4+5 = 9.

Since r% of the revenue comes from A:
r = revenue from A/total revenue * 100 = 4/9 * 100 = 400/9. This is our target.

Now we plug p=50 into the answers to see which yields our target of 400/9.
The denominator of the correct answer choice must yield a multiple of 9.
For an integer to be a multiple of 9, the sum of its digits must be a multiple of 9.
A quick scan of the denominators reveals that only D works:

125-p = 125-50 = 75. Not a multiple of 9.
250-p = 250-50 = 200. Not a multiple of 9.
375-p = 375-50 = 325. Not a multiple of 9.
500-p = 500-50 = 450, which is a multiple of 9.
625-p = 625-50 = 575. Not a multiple of 9.

The correct answer is D.

See Image Below.
Come up with your own example and solve the same problem at least twice if you got this wrong or took more that 1 minute and 45 seconds to solve this.