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Two crews can complete a job in 45 days when working together.
On their own, one crew completes the same job in 80 days.
How long would it take the other crew to complete the same job on their own?
let t = time required by the 2nd crew alone
let the completed job = 1
The two crews each do a fraction of the job, the two fractions add up to 1
multiply by 80t, results:
45t + 80(45) = 80t
3600 = 80t - 45t
3600 = 35t
t = 102.857 days, crew two alone
See if that checks out
.5625 + .4375 = 1