If the arithmetic mean of p and q is 10 and the arithmetic mean of q and r is 25, what is the value of r-p?	Choose one:
A) 20
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 = 50 - - - - eq 2 subtracting eq 1 from eq 2 we get : r - p = 30 hence , the correct answer is c"
Answer: C