Walkers 1800 m apart: when do they meet?

Xiaoming and Xiaohong start 1800 m apart and walk toward each other at 70 m/min and 50 m/min. After how many minutes do they meet?

Step-by-step

1. 70 + 50 = 120
   Moving toward each other, the relative speed is the sum of the speeds
2. 1800 ÷ 120 = 15
   Time to meet head-on = distance ÷ relative speed

Why it works

Walking toward each other, they close the distance at the sum of their speeds, so the meeting time is the total distance divided by that sum.

Common mistakes

• Subtracting the speeds, as if one were chasing the other.
• Using only one walker's speed to divide the distance.

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