Explanation:

Let's say usual speed = S
Walking at 3/4th of usual speed reaches 20 minutes
late i.e., T+20
Speed = Distance/Time
(3/4)S = D/(T+20)
So, distance is the same in both cases
So, S*T = (3/4)S*(T+20)
4T = 3T + 60
T= 60 minutes