A sports retailer ordered white and yellow tennis balls in equal number but the dispatch clerk of the tennis ball company erred and dispatched 50 extra yellow balls and making ratio of white balls to yellow balls 7/11. How many tennis balls did the retailer order originally. Choose the correct choice from the following answers:
A) 17.5
B) 130
C) 175
D) 195
E) 220

Rationale: "white : yellow = x : ( x + 50 ) = 7 : 11 - - > 11 x = 7 x + 350 - - > x = 87.5 . the total # of balls originally x + x = 87.5 + 87.5 = 175 . answer : c ."
Answer: C