The ratio of ducks and frogs in a pond is 31 : 39 respectively. The average number of ducks and frogs in the pond is 152. What is the number of frogs in the pond ?	Choose the correct choice.
A) 148 B) 152 C) 156 D) 169 E) none

Rationale: "solution : ratio of ducks and frogs in pond , = 31 : 39 . average of ducks and frogs in pond , = 152 . so , total number of ducks and frogs in the pond , = 2 * 152 = 304 . therefore , number of frogs , = ( 304 * 39 ) / 70 = 169 . answer : option d"
Answer: D