Can you measure out 9 minutes?
You have 2 sandglasses - one for 4 minutes, another for 7. Can you use them to measure out 9 minutes?
I can, it took me maybe a minute or two.
[Spoiler (click to open)]1. Turn the glasses at the same time,
In 4 minutes, there is 3 minutes left in 7-minute glass,
2. Turn empty 4-minute glass,
In 3 minutes, there is 1 minute left in 4-minute glass,
3. Turn empty 7-minute glass,
In 1 minute, there is 6 minutes left in 7-minute glass,
4. Turn 7-minute glass, with one minute now measured.
4+3+1+1
I can, it took me maybe a minute or two.
[Spoiler (click to open)]1. Turn the glasses at the same time,
In 4 minutes, there is 3 minutes left in 7-minute glass,
2. Turn empty 4-minute glass,
In 3 minutes, there is 1 minute left in 4-minute glass,
3. Turn empty 7-minute glass,
In 1 minute, there is 6 minutes left in 7-minute glass,
4. Turn 7-minute glass, with one minute now measured.
4+3+1+1