SOLUTION: A and B can do a piece of work in 72 days. B and C can do it in 120 days. A and C can do it in 90 days. In what time can A alone do it?

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Question 874737: A and B can do a piece of work in 72 days. B and C can do it in 120 days. A and C can do it in 90 days. In what time can A alone do it?
Answer by rothauserc(4718) About Me  (Show Source):
You can put this solution on YOUR website!
A + B = 72
B + C = 120
A + C = 90
therefore
B = 120 - C
C = 90 - A
B = 120 - 90 + A = 30 + A
A + 30 + A = 72
2A = 42
A = 21 days