A person goes from A to B with a speed of 30 km/h. He comesback from B to A with a speed of 20 km/h. What is his average speed forthe entire journey?

let us take the distance between A and B as LCM(30, 20) which is 60 km

now speed of the person to travel A to B = 30 km/hr

Time taken to travel from A to B = $$\frac{distance}{speed}$$ = $$\frac{60}{30}$$ = 2 hrs

now speed of the person to travel B to A = 20 km/hr

Time taken to travel from B to A = $$\frac{distance}{speed}$$ = $$\frac{60}{20}$$ = 3 hrs

average speed of the entire journey = $$\frac{ total distance}{total time taken}$$

total distance = 60 + 60 =120        ( A to B and then from B to A)

total time = 5 hrs

average speed for the entire journey=  $$\frac{ 120}{5}$$ = 24 km/hr