perplexus.info :: Just Math : Coordinating arrival

Coordinating arrival (Posted on 2015-09-15)

Alex wants to take his two sons to visit their grandmother,
living 33 kilometers away.
His motorcycle can run 25 kilometers per hour if he rides alone, but the speed drops to 20 kph if he carries one passenger and he cannot carry two.
The walking speed of each of the boys is 5 kph.

How can the three of them reach grandmother’s house in 3 hours?

Source : Russian Mathematical Olympiad (1999)

Submitted by Ady TZIDON

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Solution: (Hide)

Simultaneously with Alex driving one son towards his grandmother the other son is walking. Alex turns back prior to reaching his destination. The 1st son walks the rest of the way and Alex drives back to get the other son. If all of them are in motion – the total time will be 3 hours.
Detailed solution provided by the solvers.

Comments: ( You must be logged in to post comments.)

Subject	Author	Date
Slightly different mothod to solution.	Jer	2015-09-15 13:20:13
solution	Charlie	2015-09-15 10:57:22

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