Warning: Do not read other posters' comments unless you give up since it usually contains the answers. Clutchcity.net posters are very smart. Last riddle post for today. You are an elite spy working for the government. Your task is to infiltrate to the enemy's base which is famous for being heavily guarded. You study the map and find out a way in through a small passage with rotary fan which stops every 9 minutes. You therefore need to get in while the fan stops exactly at the 9th minute. But alas, your digital watch is broken. Fortunately, you're able to find two hourglasses able to precisely measure in hand-crafted swiss sand seven and four minutes, respectively. How can you use these two hourglasses to get in exactly at the 9th minute?
OK, start with each hourglass "topped off". Flip them both. At 4 minutes: your first hourglass is at 0 or 4 minutes (depending on which way it is flipped). The other one is at 3 or 4 minutes. Now, flip the two hourglasses again. 3 minutes later (7 minutes total), your 7M glass will be at 7/0. Your 4M glass will be at 1/3. Flip both hourglasses again. After 1 minute (8 minutes total), your 7M glass will be at 6/1. Your 4M glass will be at 0/4. Flip the 7M hourglass again. When it runs out, you're at 9 minutes. However, since you took a few minutes to figure this out, you're all screwed up and likely dead. Alternatively, just keep flipping the 4M hourglass nine times and at 36 minutes, you'll have an opening.
You meant to say flip only the 4M glass, not both hourglasses, right? If you also flip 7M glass when it's at 3/4 at the 4th minute, that means three minutes later, your 7M glass will be at 1/6. Again, you meant to say flip only the 7M glass. If you flip both glasses (when the 4M glass is at 1/3), after 1 minute, the 4M glass will be at 2/2. LOL, that's true.
You meant to say flip only the 4M glass, not both hourglasses, right? If you also flip 7M glass when it's at 3/4 at the 4th minute, that means three minutes later, your 7M glass will be at 1/6. I meant to do whatever eats up all of the 3 minutes remaining on the 7M glass. Again, you meant to say flip only the 7M glass. If you flip both glasses (when the 4M glass is at 1/3), after 1 minute, the 4M glass will be at 2/2. Same thing here. Run the $M glass down to 0 from the 1 minute that's remaining, and get the other one to 1M from 7/0. I've thoroughly confused myself on all the flipping.
Cause I sure as hell wouldn't trust a watch to jump in and risk my head being chopped off. My ass is waiting to see when that fan stops with my own damn eyes.
heh...heh... Everybody is right. Maybe my poor question has too many holes in it. How about this addition: The fan only stops at the 9th minute and won't stop again afterwards. You have to go in at the 9th minute. And the guy is both blind and deaf, so he can't see nor hear the fan.
Then he has some nerve calling himself a "spy". What kind of crappy stupid government is he working for? Iraq?
Major has actually answered it, although it may be confusing to some. Here's my attempt to clarify how to get in at exactly the 9th minute using 4-minute and 7-minute hourglasses: 1. Start by flipping both hourglasses so they run parallel. 2. At the 4th minute, the 4-minute hourglass is empty. The 7-minute hourglass has 3 minute worth of sand at the top and 4 minute worth of sand at the bottom. 3. Immediately flip back the 4-minute hourglass while still running the 7-minute hourglass. 4. After another 3 minutes (at the 7th minute), the 4-minute hourglass has 1 minute worth of sand at the top and 3 minutes worth of sand at the bottom. The 7-minute hourglass is empty. 5. Immediately flip back the 7-minute hourglass while still running the 4-minute hourglass. 6. After another minute (at the 8th minute), the 4-minute glass is empty and the 7-minute hourglass has 6 minutes worth of sand at the top and 1 minute worth of sand at the bottom. 7. Immediately flip back the 7-minute hourglass again (so the top of the glass will have 1 minute worth of sand and the bottom will have 6 minutes worth of sand) 8. In another minute (at the 9th minute), the 7-minute hourglass is empty. Time to get into the passage.
