Walking at three-fourths of his usual speed, Manav reaches his office 15 minutes later than he normally does. His usual time for the walk (in minutes) is:

Over the same distance, time varies inversely as speed, so at 3/4 of the speed he takes 4/3 of the usual time. The extra 1/3 of the usual time is 15 minutes, so the usual time is 45 minutes. 60 minutes is what he took THAT day (45 + 15), and multiplying 15 by 4/3 gives 20, which uses the delay in place of the usual time.