The Four Friends
There are four friends that need to cross a bridge to get home. Due
the age of the bridge, only two people can cross at the same time.
To make matters worse, it is night-time and there is only one torch.
The torch is always required when crossing the bridge and the torch
must be walked back and forth, it cannot be thrown, etc.
Each friend walks at a different speed and a pair must walk together
at the rate of the slower person:
Mary takes 1 minute to cross
Bill takes 2 minutes to cross
Carl takes 5 minutes to cross
Susan takes 10 minutes to cross
In what sequence can they travel to all get across in the quickest
time?