A distance of 320 km is traveled in three parts first 100 km at a speed of 20 km/hr, for the next 100 km it increases its speed by 25%, and for the last 120 km, it increases its speed further by 20%. Find the average speed of the whole journey. (approx.)?

Option 2 : 25 km/hr

Given:

For the first 100 km its speed  = 20 km/hr

Formula used :

Average speed = Total distance / total time taken

Calculations :

Time taken to travel first 100 km = 100/20 = 5 hr

⇒ For the next 100 km it increases its speed by 25%

= 20 + 25% of 20

= 25 km/hr

Time taken to travel next 100 km = 100/25 = 4 hr

⇒ For the next 100 km it increases its speed by 20%

= 25 + 20% of 25

= 30 km/hr

Time taken to travel last 120 km = 120/30 = 4 hr

⇒ Total time = (5 + 4 + 4) hr

⇒ Average speed = 320/(5 + 4 + 4) = 320/13

⇒ 24.62 km/hr ≈ 25 km/hr

∴ The average speed of the whole journey is 25 km/hr.