The contents of two vessels containing copper and tin in the ratio 1:2 and 2:5 are mixed in the ratio 1:4. The resulting mixture will have copper and tin in the ratio? Pick one
A) 15 : 32
B) 21 : 28
C) 31 : 74
D) 33 : 37
E) 41 : 92

Rationale: "the ratio of copper and tin the new vessel = ( 1 / 3 * 1 / 5 + 2 / 7 * 4 / 5 ) : ( 2 / 3 * 1 / 5 + 5 / 7 * 4 / 5 ) = 31 / 105 : 74 / 105 = 31 : 74 answer is c"
Answer: C