A thief running at $7\text{km/h}$ is chased by a policeman whose speed is $12\text{km/h}$. If the thief is $280\text{m}$ ahead of the policeman, then the time required for the policeman to catch the thief will be:

The initial distance separating the policeman and the thief is $d=280\text{m}$.
Since both are running in the same direction, the relative speed ($v_{\text{rel}}$) is:

$$v_{\text{rel}}=12\text{km/h}-7\text{km/h}=5\text{km/h}$$

Convert relative speed into meters per second ($\text{m/s}$):

$$v_{\text{rel}}=5\times \frac{5}{18}=\frac{25}{18}\text{m/s}$$

The time required to catch the thief ($t$) in seconds is:

$$t=\frac{\text{Distance}}{\text{Relative Speed}}=\frac{280}{\frac{25}{18}}=\frac{280\times 18}{25}\text{seconds}$$

Convert the time into minutes by dividing by 60:

$$t=\frac{280\times 18}{25\times 60}\text{minutes}=\frac{28\times 3}{25}\text{minutes}=\frac{84}{25}\text{minutes}=3\frac{9}{25}\text{minutes}$$