A man walking at three-fourth of his usual speed, reaches his office 4 minutes late. How much time (in minutes) does he usually take to reach his office?

Since the distance to the office remains constant, the speed and time are inversely proportional to each other.
Ratio of speeds (Usual : New)=1:
4
3
​
=4:3
Ratio of times taken (Usual : New)=3:4
Let the usual time be 3t and the new time be 4t.
The difference between the new time and usual time is given as 4 minutes:

4t−3t=4⟹t=4 minutes
The usual time taken by him to reach his office is:

Usual time=3t=3×4=12 minutes