Given: Walking as 3/5 of his usual speed, a man reaches his office 10 minute late Formula Used: speed = distance/time Calculation: Let the usual speed and usual time be 's' and 't' respectively and distance remains constant in both the cases ⇒ s × t = 3s/5 × (t + 10) ⇒ st = 3st/5 + 6s ⇒ 2st/5 = 6s ⇒ t = 15 minutes ∴ required usual time = 15 minutes