If the arithmetic mean of p and q is 10 and the arithmetic mean of q and r is 24, what is the value of r-p? Choose the correct choice from the following answers:
A) 28  B) 10  C) 30  D) 40  E) 5

Rationale: "arithmetic mean expression for p and q : ( p + q ) / 2 = 10 ; p + q = 20 - - - - eq 1 arithmetic mean expression for q and r : ( q + r ) / 2 = 20 ; q + r = 48 - - - - eq 2 subtracting eq 1 from eq 2 we get : r - p = 28 hence , the correct answer is a"
Answer: A