A dog start chasing to a cat 2 hour later. It takes 2 hours to dog to catch the cat. If the speed of the dog is 30 km/h. what is the speed of cat?

Answer: Option C

Let speed of the cat be S km/h.
Time taken = $$\frac{\text{Distanced}\text{ advanced}}{\text{Relative}\text{ speed}}$$
2 = $$\frac{2\times \text{S}}{30-\text{S}}$$
S = 15 km/h