Four puzzles. Give each one a real minute before moving on.
1. The shop. A shopkeeper has ten identical-looking bags of coins. Nine contain coins weighing 10 grams each; one contains coins weighing 9 grams each. He has a digital scale and may use it exactly once. How does he find the light bag?
2. The clock. A clock strikes six times in five seconds. How long does it take to strike twelve?
3. The river. Two men on opposite banks. Each has a boat that carries only one person. Neither can swim. Both cross. How?
4. The number trap. A bat and a ball cost 110 rupees together. The bat costs 100 rupees more than the ball. What does the ball cost?
Answers.
1. Take 1 coin from bag one, 2 from bag two, and so on up to 10 from bag ten. Weigh them all together. If all were 10g the total would be 550g. Every gram short tells you the bag number — 3g short means bag three.
2. Eleven seconds, not ten. Six strikes have five gaps; twelve strikes have eleven. Each gap is one second.
3. They are on opposite banks. One crosses, gets out; the other uses the same boat to cross the other way. Nothing says they cross at the same time.
4. Five rupees. Almost everyone says ten — but then the bat would be 110 and the total 120.
