Rehman walks at $8\text{km/hr}$ and Roma cycles at $13\text{km/hr}$ towards each other. What was the distance between them when they started if they meet after 36 minutes?

Since Rehman and Roma are moving towards each other, their relative speed is the sum of their individual speeds:
$$\text{Relative Speed}=8\text{km/hr}+13\text{km/hr}=21\text{km/hr}$$
The time taken to meet is 36 minutes. Convert this time into hours:
$$\text{Time}=\frac{36}{60}\text{hours}=0.6\text{hours}$$
The initial distance between them is equal to the distance covered at their relative speed:
$$\text{Distance}=\text{Relative Speed}\times \text{Time}$$
$$\text{Distance}=21\times 0.6=12.6\text{kms}$$