By walking at 3/4 of his usual speed , a man reaches his office 20 minutes later than usual . this usual time is?

At 3/4 of the usual speed, the time taken is 4/3 of the usual time ∴ 4/3 of usual time - usual time = 20 min ⇒ 4x/3 - x = 20 ⇒ x/3 = 20 ∴ x = 60 min.