A does 80% in 20 days, so A does 100% in 25 days. A's rate is 1/25 of the job per day. The remaining 20% work is done by A and B in 3 days. (A+B) rate = (1/5 job) / 3 days = 1/15 job/day. B's rate = (A+B) rate - A's rate = 1/15 - 1/25 = 2/75. So B alone would take 75/2 = 37.5 days.