# The Collapsing Bridge Puzzle - Solution

## The Puzzle:

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?

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?

## Our Solution:

A and B cross first using up 2 minutes

A comes back making it 3 minutes

C and D cross making it 13 minutes

then B crosses back over making it 15 minutes

And finally A and B cross together to make it 17 minutes!

Ashwyn Sam kindly provided us with an alternative solution:

A and B cross first using up 2 minutes

B comes back making it 4 minutes

C and D cross making it 14 minutes

then A crosses back over making it 15 minutes

And finally A and B cross together, also for a total of 17 minutes.

A comes back making it 3 minutes

C and D cross making it 13 minutes

then B crosses back over making it 15 minutes

And finally A and B cross together to make it 17 minutes!

Ashwyn Sam kindly provided us with an alternative solution:

A and B cross first using up 2 minutes

B comes back making it 4 minutes

C and D cross making it 14 minutes

then A crosses back over making it 15 minutes

And finally A and B cross together, also for a total of 17 minutes.

Puzzle Author: Tom Rosenzweig