explorers’ club

So I think I am gonna start posting weekly riddles. Its fun, its educating, and I like the challenge. And I consider you, my peers, to be like-minded so I assume you enjoy riddles too. Please keep in mind that others might want to challenge themselves so please refrain from posting your answers in the comments. Instead, I will post the answer on next week’s weekly riddle.

This week: The Runners…

The facts:

• There are four runners.
• It is dark.
• There is only one flashlight.
• There is a bridge spanning a huge crevasse.
• The bridge can only sustain two people at any one time (its old, long, and creeky).
• It is necessary to traverse the bridge with a flash light.
• The crevasse is too wide and far to toss the flash light across, or otherwise transport it by any means other than by crossing the bridge.
• Runner A can cross the bridge in 1 minute.
• Runner B can cross the bridge in 2 minutes.
• Runner C can cross the bridge in 5 minutes.
• Runner D can cross the bridge in 10 minutes.
• This is set in reality meaning the runners have no James Bond-ish gadget for such an occasion.

The goal:

• What is the shortest amount of time to get all four runners across the bridge?