80% of a work is done by one person in 20 days. Then Another person came and they together finish the remaining work in 3 days. How long another person alone would take to do the whole work?

Whole work is done by first person in 20 * (5/4) = 25 days

so ((1 - (4/5)) = 1/5

So both three day's work = 1/5

first person 1 days work = 1/25,

both 1 days work = 1/15,

second person 1 days work = (1/15 - 1/25)

                                       = 4/150 = 2/75

second person would do the work in 75/2

                                                = 37.5