How do they all cross before the bridge collapses?
A bridge will collapse in 17 minutes.
4 people want to cross it before it will collapse. It is a dark night and there is only one torch between them.
Only two people can cross at a time.
“A” takes a minute to cross.
“B” takes 2 minutes.
“C” takes 5
and “D” takes 10 minutes
How do they all cross before the bridge collapses?
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first A+B will cross – time taken 2mins
B will stay on the other side and A comes back – 1 min (total 3mins elapsed)
Then C+D will cross – time taken to cross 10mins (total 13 mins elapsed)
B comes back with torch – time taken to cross 2mins (total 15mins elapsed)
Finally A+B will cross – time taken to cross 2mins (total 17 mins elapsed)