A man reaches his office 20 min late, if he walks from his home at 3 km per hour and reaches 30 min early if he walks 4 km per hour. How far is his office from his house ?

Let distance = x km. Time taken at 3 kmph : dist/speed = x/3 = 20 min late. time taken at 4 kmph : x/4 = 30 min earlier difference between time taken : 30-(-20) = 50 mins = 50/60 hours. x/3- x/4 = 50/60 x/12 = 5/6 x = 10 km.