The key here is, as someone noted, to understand that the truck needs 1.5 min to do 1km (3/2 minutes since it's travelling at 2/3 km per minute, aka 40km/h)
Given the fact above, you can easily see that, if in 1.5 min the truck does 1000m, in the same time the boy, moving at 1000m every 20 min (aka 3km per hour), would have done 1000/20*1.5=75m.
So the distance between the truck and the boy when the truck breaks down is actually 1000m-75m=925m
We know that the boy is travelling at 50m/min (aka
...
Given the fact above, you can easily see that, if in 1.5 min the truck does 1000m, in the same time the boy, moving at 1000m every 20 min (aka 3km per hour), would have done 1000/20*1.5=75m.
So the distance between the truck and the boy when the truck breaks down is actually 1000m-75m=925m
We know that the boy is travelling at 50m/min (aka
...
