if A fills a tank in 30mins,and B fills the same tank in 20mins,and C emptys it in 40min ,if A,B,C works together how much time will the tank take to fill?
Choose the correct choice from the following answers.
A) 15.14 mins B) 16.14 mins C) 17.14 mins D) 18.14 mins E) 19.14 mins

Rationale: 1 / 30 + 1 / 20 - 1 / 40 = 7 / 120 so total time = 120 / 7 = 17.14 mins answer : c
Answer: C