Correct Answer - (b)
Let efficiency of B is x
Days are inversaly proportional to efficiency.
According to question
`3x-x=40`
`2x =40`
`x=20`
Time taken by `A = x days = 20 days`
Time taken by `B=3x` days `= 3 xx 20= 60 ` days
Time taken by A and B togeter `= (20xx60)/(80 = 15` days