Kinesys materializes on the edge of a ravine on Veritas, in the middle of the night. On the far side glow the lights of a Veritasian patrol’s base camp. On this side stand four tired, mud-caked soldiers, back from an exercise that went terribly: the maps were perfectly truthful, but nobody had mentioned they were of another continent.
Between the two sides is a rickety bridge. The sign at the entrance reads: “This bridge holds two people. Not three.” The base camp will close its gates in exactly 17 minutes.
Liz would like to help, but Kinesys can’t even teleport itself to the right place. Dord protests against whoever builds such flimsy bridges. M00N switches on her eye and cheers everyone on.
The Riddle
There are 4 soldiers who need to reach their base camp on the other side of a bridge, but:
- the bridge can hold only two people at a time;
- it’s dark and, since the bridge is rickety, a flashlight is needed to cross it; naturally, the 4 soldiers have only one;
- the soldiers are in different physical shape: soldier A takes 1 minute to cross the bridge, B takes 2, C takes 5, and D takes 10;
- when two soldiers cross the bridge together with the flashlight, they move at the pace of the slower one;
- the soldiers have only 17 minutes for all 4 of them to be on the other side of the bridge.
How can they do it?
Hint
The two slowest soldiers can’t afford to cross on two separate trips. Think carefully about whom to trust with bringing the flashlight back.
Solution
- A and B cross the bridge (2 minutes) and B comes back with the flashlight (2 minutes): 4 minutes total.
- C and D cross the bridge (10 minutes) and A, who was already on the other side, comes back with the flashlight (1 minute): 15 minutes total.
- A and B cross the bridge again (2 minutes): everyone is safe in 17 minutes.
The trick is to have the two slowest soldiers cross together, so their times don’t add up, and to use the fastest ones to bring the flashlight back.