There are three types of zinc-aluminum alloy, \(M_1, M_2\) and \(M_3,\) with \(10\%, 40\%\) and \(70\%\) concentration of zinc respectively. If \(p\) grams of \(M_1,\) \(q\) grams of \(M_2\) and \(r\) grams of \(M_3\) is mixed to form \(p +q+ r\) grams of mixture with \(25\%\) zinc concentration, what is the value \(r\) in terms of \(p\) and \(q?\)
A. \(\dfrac{p-q}3\)
B. \(\dfrac{p-q}2\)
C. \(\dfrac{p+q}2\)
D. \(\dfrac{p+q}3\)
E. \(\dfrac{p+2q}3\)
Answer: A
Source: e-GMAT