A man runs at a speed of $6\text{km/h}$. He takes 30 minutes rest at the end of every $4\text{km}$. How long (in hours) will he take to cover $15\text{km}$?

Total running distance = $15\text{km}$ Running speed = $6\text{km/h}$ Actual running time:
$$\text{Running time}=\frac{\text{Distance}}{\text{Speed}}=\frac{15}{6}=2.5\text{ hours}$$
The man rests after every $4\text{km}$. Let's calculate the number of rest intervals:
$$15÷4=3\text{ with a remainder of }3$$
This means he stops at the $4\text{km}$, $8\text{km}$, and $12\text{km}$ marks. He does not stop at $16\text{km}$ because he reaches his destination at $15\text{km}$.Number of rests = 3Total rest time = $3\times 30\text{ minutes}=90\text{ minutes}=1.5\text{ hours}$ Total time taken = Running time + Rest time
$$\text{Total time}=2.5+1.5=4\text{ hours}$$