A cheetah at 28 m/s chases a gazelle running at 22 m/s in a straight line…

Step 1. Closing speed. $28\ \text{m/s} - 22\ \text{m/s} = 6\ \text{m/s}$.

Step 2. A 60 m lead. $\dfrac{60\ \text{m}}{6\ \text{m/s}} = 10\ \text{s}$, within the $15\ \text{s}$.

Step 3. A 100 m lead. In $15\ \text{s}$ the cheetah gains $6\ \text{m/s} \times 15\ \text{s} = 90\ \text{m}$, so the gazelle is still $100\ \text{m} - 90\ \text{m} = 10\ \text{m}$ ahead.