A man driving his bike at 24 kmph reaches his office 5 minutes late... - with answer

A man driving his bike at 24 kmph reaches his office 5 minutes late. Had he driven 25% faster on an average he would have reached 4 minutes earlier than the scheduled time. How far is his office?

Answer: 18 km

Let x km be the distance between his house and office.
While travelling at 24kmph, he would take $\frac{x}{24}$ hours.
While travelling at 25% faster speed,
i.e. $24+25\%\text{of}24=24\times \left(\frac{1}{4}\right)=30$ kmph, he would take $\frac{x}{30}$ hours.
Now as per the problem, time difference = 5 min late + 4 min early = 9 min
⇒ $\frac{x}{24}-\frac{x}{30}=9\text{min}$
⇒ $x=$ 18 km