A AND B CAN DO A WORK IN 12 DAYS, B AND C IN 15 DAYS, C AND A IN 20 DAYS. IF A, B AND C WORK TOGETHER, THEY WILL COMPLETE THE WORK IN: Answers:  A) 5 days  B) 7 5 / 6 days  C) 10 days  D) 15 2 / 3 days  E) none of these

Rationale: "( a + b ) ' s 1 day ' s work = 1 / 12 , ( b + c ) ' s 1 day ' s work = 1 / 15 ; ( a + c ) ' s 1 day ' s work = 1 / 20 adding , we get : 2 ( a + b + c ) ' s 1 day ' s work = ( 1 / 12 + 1 / 15 + 1 / 20 ) = 12 / 60 = 1 / 5 ( a + b + c ) ' s 1 day ' s work = 1 / 10 so , a , b and c together can complete the work in 10 days . correct option : c"
Answer: C