Richa travels from A to B at the speed of 15 km/h, from B to C at 20 km/h, and from C to D at 30 km/h. If AB = BC = CD, then find the Richa's average speed.

Answer: Option D

Let AB = BC = CD = 60 km (LCM of 15, 20 and 30)
Total distance = 3 × 60 = 180 km
Time taken by Richa to travels from A to B = $$\frac{60}{15}$$ = 4 hrs.
Time taken by Richa to travels from B to C = $$\frac{60}{20}$$ = 3 hrs.
Time taken by Richa to travels from C to D = $$\frac{60}{30}$$ = 2 hrs.
Total time taken by Richa to travels from A to D = 4 + 3 + 2 = 9 hrs.
∴ Average speed = $$\frac{180}{9}$$ = 20 km/hr.