A and B can do a piece of work in 15 days; B and C in 12 days; C and A in 20 days. How many days will be taken by A, B and C working together to finish the work?
Number of days required by A and B to finish the work : 15 days
Number of days required by A and B to finish the work : 12 days
Number of days required by A and B to finish the work : 20 days
Work done by A and B in one day: 
Work done by B and C in one day: 
Work done by C and A in one day: 
Work done by (A and B) , (B and C ) and (C and A) in one day that is work done by
(A +B) + (B + C) + (C + A) = 2(A+B+C) = 
Work done by A, B and C = 
= 
They can do the work in 10 days
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