A man divides his total route of journey into three equal parts and decide to travel the three parts with speed of 15 km/h. 10 km/h and 5 km/ h respectively. Find his average speed of during journey.

If three equal distances are covered at different spees of A, B and C km/h, respectively, then average speed during whole journey
= 3ABC/(AB + BC + CA) km/h
∴ Required average speed
= (3 x 15 x 10 x 5) / (15 x 10 + 10 x 5 + 15 x 5)
= (3 x 15 x 10 x 5)/(150 + 50 + 75)
= (3 x 15 x 10 x 5)/275
= 8.18 km/h