A and B can do a piece of work in `12` days. B and C in `15` days. C and A in `20` days, A alone take to finish the job ?

Explanation

Let the total work is `60` unit
because `LCM `of `12,15,16` is`60`
Work done by `A` and `B` is`60/12 = 5`unit per day.
work done by `B` and `C `is `60/15 = 4` unit per day.
work done by `C` and` A` is`60/20 = 3` unit per day.
Now `A, B` and `C` can do the work in `12/2 = 6` unit per day .
And `B` and `C` work `4` unit per day.
Now A work per day is unit work of `(A + B + C) - (B+ C) = 6 - 4 = 2` unit per day
` A` alone will finished the work in `60/2 = 30` days.