Car chase: 40 km behind, 80 and 60 km/h

A car starts 40 km behind another car, and both drive in the same direction at 80 km/h and 60 km/h. After how many hours does the faster car catch up?

Step-by-step

1. 80 − 60 = 20
   Moving the same way, the relative speed is the difference of the speeds
2. 40 ÷ 20 = 2
   Catch-up time = distance ÷ relative speed

Why it works

The faster car gains on the slower one at the difference of their speeds, so the time to close a 40 km gap is the gap divided by that difference.

Common mistakes

• Adding the speeds instead of subtracting them.
• Dividing 40 km by 80 km/h.

Related

meeting-problem meeting-walkers-2400m-90-60 catch-up-cars-45km-90-75