A and B can do a piece of work in 7 days, B and C in 8 days, C and A in 9 days. How long will C take to do it? Choose the correct choice from the following: A) 14.1 days B) 18.8 days C) 21.4 days D) 24.3 days E) 26.5 days

Rationale: "2 c = 1 / 8 + 1 / 9 – 1 / 7 = 47 / 504 c = 47 / 1008 = > 1008 / 47 = 21.4 days the answer is c ."
Answer: C