A girl claps 14 m from a wall. Will she hear a separate echo? …

Step 1. $t = \dfrac{2 \times 14\ \text{m}}{340\ \text{m/s}} = 0.082\ \text{s}$.

Step 2. That is less than $0.1\ \text{s}$, so the echo mixes with the clap.

Step 3. For $0.1\ \text{s}$: $d = \dfrac{340\ \text{m/s} \times 0.1\ \text{s}}{2} = 17\ \text{m}$.