## Do You Know?

What is black when you get it, red when you use it, and grey when you’re through with it?

The answer is not this picture.

## 9-Minute Rule

You have 2 sand-filled hourglasses. One is a 4-minute timer, and the second is a 7-minute timer. How do you use both timers to measure 9 minutes?

Post your answers in the comments section. We’ll update with answers soon…

1. coal

2. Start the 7-minute timer first. When that’s done, start the 4-minute timer. When the 4-minute timer is half-finished, that’s about 9 minutes.

Coal

Start both timers at the same time.

When the four minute runs out, flip it again.

When the seven minute runs out, begin your nine minute interval.

Let the remaining minute fall through the four minute and run it twice

more. Voila, 9 minutes.

Start both timers. When the 4 minute one finishes, flip it. Then, when the 7 minute one finishes flip that. One minute later the second cycle of the four minute timer will complete, at which point you’ll have measured 1 minute on the 7 minute timer, so flip that and it will complete in one more minute–nine minutes after starting.

Start both timers. When the 4-minute runs out, flip it over (but leave the 7-minute one running). When the 7-minute runs out, tip the 4-minute one to stop it (leaving 3 minutes in one side and 1 minute in the other).

You are now ready to time 9 minutes: flip the 4-minute time to finish the 1 remaining minute, then flip the 4-minute timer twice more for 8 more minutes total.

(I’m sure there’s a more elegant solution, but this is the best I could come up with!)