Below (after the reminder) is the answer to the Problem 5 of the lesson Some logic problems (the topic Miscellaneous word problems
of the section Word problems in this site).
Problem 5 formulation (reminder)
It takes 15 minutes to boil an egg.
How could you boil the egg having two hourglasses that measure 7 and 11 minutes?
Start running both 11-minutes and 7-minutes hourglasses simultaneously.
When the 7-minutes hourglass emptied, start to boil the egg.
When 11-minutes hourglass emptied, turn it over and run again. Continue to boil the egg.
When 11-minutes hourglass emptied again, boiling of the egg is completed.