A Problem
by bellasemplicita
Recently someone allowed me to borrow a book entitled “The Chicken From Minsk”. It contains many different traditional Russian math and logic problems, some of which are very irritating. If you solve the problem below post a comment on the explanation. For those of you who don’t feel like solving it, I will post the solution next week. Have fun!
Grandfather’s Breakfast
Grandfather is a very hard-boiled customer. In fact, his eggs must be boiled for exactly 15 minutes, no more, no less. One day he asks you to prepare breakfast for him, and the only timepieces in the house are two hourglasses. The larger hourglass takes 11 minutes for all the sand to descend; the smaller, 7 minutes. What do you do? (Grandfather grows impatient!)
Hint: Think carefully about the first step. The logic is unique!
bellasemplicita 
So what is the solution? I can’t figure it out…
I am sorry that it took me so long to post an answer!
srm
Here is a clue: if you start both timers at the same time, when one ends the other will have 4 minutes remaining.
Before beginning to boil the egg, run the eleven and the seven timers together until the 11 timer is left with (11-7) 4 minutes. Then put the eggs to boil for the remaining 4 minutes of the 11 timer, and then turn it to run for another 11 minutes. Therefore the egg would have run for 11+4=15 minutes. 🙂