Chapter 2 - Kinematics: Description of Motion - Learning Path Questions and Exercises - Exercises - Page 61: 22

a). $12.5s$ b). $56.25m$ from starting point of runner with speed 4.5 m/s.

Their relative velocity or the velocity of approach = $4.5+3.5=8 m/s$ Initial distance = 100m. So, time taken to cover this distance is $=\frac{100}{8}=12.5 s$ In 12.5 s, the runner with speed 4.5 m/s would cover $=4.5\times 12.5 = 56.25m$