You have two hourglasses: one measures exactly 7 minutes, and the other measures exactly 11 minutes. How can you use only these two sand timers to measure an interval of exactly 15 minutes?
Start both timers. When the 7-minute timer ends, restart it. When the 11-minute timer ends, turn the 7-minute timer over again.
At minute 11, the 7-minute timer has been running for 4 minutes in its second cycle (leaving 4 minutes of sand at the bottom). Flipping the 7-minute timer over at minute 11 causes those 4 minutes of sand to run back down, adding exactly 4 minutes to the elapsed 11 minutes for a total of 15 minutes.
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
Maya, Nina, and Omar stand in a single line facing forward: Maya at the back, Nina in the middle, Omar at the front. There are 3 white hats and 2 black hats available. Maya can see Nina and Omar's hats. Maya says she doesn't know her hat color. Nina, hearing this, realizes she knows her own hat color. If Omar is wearing a black hat, what color is Nina's hat?
In a secure facility, a guard checks security logs every time someone enters or leaves. Alice enters at 8:00 AM, Bob leaves at 9:00 AM, Charlie enters at 10:00 AM, and Diana leaves at 11:00 AM. No one else enters or leaves. If the building started completely empty at midnight, how many people are inside the facility at 12:00 PM?
A shepherd needs to cross a river with a wolf, a goat, and a basket of cabbages using a small boat that can only carry the shepherd and one item at a time. However, the wolf cannot be left alone with the goat, and the goat cannot be left alone with the cabbages. To successfully cross the river while keeping everything safe, what is the absolute minimum number of one-way boat trips the shepherd must make from start to finish?
A man pushes his car up to a hotel and tells the owner he is bankrupt. Why?
Three signal lights flash at regular intervals. The Red light flashes every 2 seconds, the Green light every 3 seconds, and the Blue light every 4 seconds. If all three lights flash simultaneously at noon, how many seconds will pass before they all flash together again?
I am a bunch of flowers wrapped in paper and given to someone special. What am I?