The initial solution most people come up with is the most obvious one: to use the fastest person, in our family, son Ben, to accompany his family members over the bridge one by one and to go back with the torch each time. This would take 10 + 1 + 5 + 1 + 2 = 19 minutes.

Easy-peasy, no? But is that it? Would Microsoft that effortlessly recruit people? Surely, not!

What seems self-evident: Ben’s participation in each crossing, is actually wrong. Think of it differently. Would it not be more useful to put the slowest members of the family together (mum and Leah) to reduce time? Sure, but if they cross first, one of them needs to go back with the torch right? So, no, this can’t be the solution. But what if Ben and his father go first, which takes them 2 minutes. Ben returns with the torch, which adds another minute, Leah and her mum now cross together, in 10 minutes. The father goes back with the torch in 2 minutes and finally, him and Ben do the final crossing in again 2 minutes. So total is:

2 + 1 + 10 + 2 + 2 = 17

Hooray!

Clever, isn’t it? It even makes more sense if you would exaggerate the crossing times of Leah and her mum, let’s say it would take them 1 hour and 1.5 hours respectively. Now, it seems obvious to put these two together in one crossing to reduce time.

Reference:

Illustration by Vecteezy accessible at https://www.vecteezy.com