Feeling dim (maths problem)

[Psymonkee]

No it's not my homework

Earlier a basketball player tweeted this link about a competition to meet him (US only, dammit ) by solving the problem on the page.

Am I being particularly dim not being able to spot the solution?

http://www.nba.com/spurs/community/manus_mathletes

 

[Ahleckz]

1 & 2 cross bridge = 2 minutes.
1 crosses back = 1 minute.
3 & 4 cross bridge = 10 minutes.
2 crosses back = 2 minutes.
1 & 2 cross bridge = 2 minutes.

Total time = 17 minutes.