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A and B together can do work W  in 12 days.
Productivity of A and B together in one day :  W /12   per day

A along can do work W in 20 days.
productivity of A in one day :       W/20

Productivity of B in one day:   W /12 - W /20 =  W/30 

So B alone can do the work W  in  W / (W /30)  =  30 days.

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