You have two hourglasses: one measures 4 minutes and the other measures 7 minutes. How can you measure an exact total span of 9 minutes using only these two timers?
Start both hourglasses together. When the 4-minute glass empties (at min 4), flip it. When the 7-minute glass empties (at min 7), flip it. At min 8, the 4-minute glass empties again; flip the 7-minute glass back. It will run for 1 minute, reaching 9 minutes.
At minute 7, flipping the 7-minute glass lets sand flow for 1 minute until minute 8 (when the 4-minute glass finishes). Flipping the 7-minute glass back at minute 8 isolates that 1 minute of accumulated sand, which takes exactly 1 more minute to drain out at minute 9.
About this riddle
Logic riddles give you a few facts and ask what must be true. List what you know, cross out what cannot be, and see what is left.
Difficulty: Medium. More like this: Logic riddles with answers or try Einstein's Logic Grid for a different kind of puzzle.
More logic riddles
A shepherd needs to cross a river with a wolf, a sheep, and a cabbage. His boat can only carry himself and one item at a time. If left unattended, the wolf will eat the sheep, and the sheep will eat the cabbage. How many one-way river crossings must the shepherd make in total to transport all three safely?
A row goes: picnic basket, picnic basket, parade float, picnic basket, picnic basket, parade float, picnic basket, picnic basket. What comes next?
Five people—Amy, Brad, Carl, Dana, and Evan—participate in a spelling bee. Amy beats Brad. Carl beats Dana. Brad beats Carl. Evan finishes immediately ahead of Amy. Who wins last place (5th place)?
You meet two islanders, X and Y. One always tells the truth and one always lies. Person X says: 'At least one of us is a liar.' What are Person X and Person Y?
Four secure digital vaults are lined up in a row. Vault A is to the right of Vault B. Vault C is to the left of Vault B. Vault D is to the right of Vault A. Which vault is second from the left?
A digital clock displays times in 12-hour format without leading zeros (e.g., 1:05 or 12:30). Between 12:00 PM and 11:59 PM, how many times will every digit on the display be identical?