ratio of time taken by A and B to complete the work alone = 1: 1.25 = 4x : 5x

As A complete half of the work alone and B complete 5% of the work alone, so they will complete remaining 45% of work in 4 days .

So time taken by them to complete the whole work together = 4 * 100/45 = 80/9

Or (4x*5x)/(4x+5x) = 80/9

x = 4

Thus time taken by B to complete the work alone = 5x = 20 days