Easy riddle

[Passions]

You have two hourglasses--a 4-minute glass and a 7-minute glass. You want to measure 9 minutes. How do you do it?

 

[Total Refected Power]

Let the 7 min run full and the 4 min go half-way. 9 minutes.

Hey, its only Monday and my brain is fries already.

 

[Demon-Xanth]

Use your watch

 

[piku]

Well if you take 3 7 minutes and subtract 3 4 minutes you get 9... but you cant measure time that way.

 

[Namuna]

Alright here we go...

1. Start both the 7min and the 4min
2. When the 4min is empty (there's 3min left in the 7min), flip the 4min
3. When the 7min is empty (there's now 1min left in the 4min)

Ah crap, comes out to only 8 minutes.

 

[Monel Funkawitz]

For the answer!! Don't read!!







To time nine minutes with a four minute and seven minute hourglass:
Start both hourglasses simultaneously.
When the four minute runs out, start it again.
When the seven minute runs out, begin timing nine minute interval.
Let the remaining minute fall through the four minute and run it twice
more.

7-4=3
4-3=1
1+4+4=9

 

[Triumph]

i don't know.

 

[Sacotool]

Despite Monel's solution, I still don't fully understand it, hehe

 

[Monel Funkawitz]

Good... cause I don't either.