Two cars 900 km apart: 100 and 80 km/h

Two cars start 900 km apart and drive toward each other at 100 km/h and 80 km/h. After how many hours do they meet?

Step-by-step

1. 100 + 80 = 180
   Moving toward each other, the relative speed is the sum of the speeds
2. 900 ÷ 180 = 5
   Time to meet head-on = distance ÷ relative speed

Why it works

Driving toward each other, the two cars close the gap at the sum of their speeds, so the meeting time is the total distance divided by that sum.

Common mistakes

• Subtracting the speeds instead of adding them.
• Dividing 900 km by only one car's speed.

Related

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