Whole work is done by A in  \( \left(20\times\frac{5}{4}\right)\) = 25 days.

Now,  \( \left(1-\frac{5}{4}\right)\) i.e.,  \(\frac{1}{5}\)   work is done by A and B in 3 days.

Whole work will be done by A and B in (3 x 5) = 15 days.

A's 1 day's work = \(\frac{1}{25}\)  ,  (A + B)'s 1 day's work = \(\frac{1}{15}\)

So, B's 1 day's work = \( \left(\frac{1}{15}-\frac{1}{25}\right)=\frac{4}{150}=\frac{2}{75}\)

So, B alone would do the work in  \( \frac{75}{2}.\)  =  \( 37^{\frac{1}{2}}\)  days.