If Afnan drives at 4/5th of his usual speed to his office, he is 6 minutes late. What is his usual time to reach his office?

A. 36
B. 24
C. 30
D. 18

Let t be his usual time to reach his office and v be his usual speed.
v = d/t ……….(d is the distance Afnan travels while going to his office)
vt = d
At v1 = 4v/5 ; t1 = t + 6
4v/5 = d/(t + 6)
4v/5* (t + 6) = d
4v/5* (t + 6) = vt
On solving we get,
t = 24 minutes