Neha and Monica walking to school

Ratio of time taken = 40 : 30 = 4 : 3. Since distance is constant, ratio of speeds = 3 : 4. Let Neha's speed be $3v$ and Monica's speed be $4v$.

In 5 minutes, Neha covers a distance of $3v \times 5 = 15v$. Relative speed of Monica with respect to Neha = $4v - 3v = v$.

Time required for Monica to overtake Neha = $\frac{\text{Initial distance}}{\text{Relative speed}} = \frac{15v}{v} = 15$ minutes.