Riddle:
A recipe requires 15 minutes to cook. You have a 7 minutes hourglass and an 11 minutes hourglass with you. How will you measure 15 minutes using them?
Show Hint
Hint:
Remember what happens when you turn an hourglass, it doesn't start from the beginning but from where it got to...
Show Answer
Answer:
You turn the 7 minute one and the 11 minutes one at the same time. When the 7 one finishes, turn it again. When the 11 one finishes, you know that 4 minutes have passed, with 4 minutes to get to the desired 15, all you need to do now is turn the 7 minute hourglass again since it counted 4 minutes, and when it runs out it'll be exactly 15 minutes.