Bridge and torch
Get all four people to the far bank in the smallest possible total time.
▤ Your first notebook · Statement, rules and controls
The challenge
Four people need to cross a bridge at night. Alone, they take 1, 2, 5 and 10 minutes. They share one torch, which must travel on every crossing, including returns.
- One or two people may cross at a time.
- A pair takes as long as its slower member. Return trips also add to the total.
- The torch cannot be thrown or return by itself: someone must carry it.
How to play
Select one or two people on the bank with the torch, then press “Cross”. The board moves the group and adds its time. Undo crossings or start again to improve your plan.
Hints are optional. Reading an explanation does not complete the level: test your strategy on the board.
Think it through together
One person proposes a strategy; the other asks “why?” and looks for a case where it fails. Then swap roles. No names or speed competition needed.
Before viewing the explanation: what do we know, what are we assuming, and how would we check the result?