Three people have to carry five boxes a distance of 120m from point A to point B. They each travel at a speed of 1m/s when carrying a box, or 2m/s when returning empty-handed for another box. Each person can carry only one box at a time.

What is the minimum time it will take them to carry all five boxes to point B?

*People often ask where I get the ideas for my puzzles. This one has a typically mundane origin: I was bringing grocery shopping in from the car with my two children. There was more than we could manage in one trip so return trips were necessary. I started to wonder whether dropping groceries partway to the kitchen might be more efficient. We didn’t implement such a solution, but it did get me thinking about how I could reformulate the question as a puzzle, so here we are.*