A and B both together can do a work in 20 days. B alone can do it in 30 days. In how many days A alone can do it

One day work of A & B is $=\frac{1}{20}$

One day work of B is $=\frac{1}{30}$

One day work of$A=\frac{1}{20}-\frac{1}{30}$

So, A alone can finish the work in 60 days