Puzzle asked in Interview

Question

There are 2 hour-glasses given to you which measures 3 and 7 hours respectively. You have to measure exactly 8 continuos hours using them.

I know that the solution exists for measuring 9 hours. I want to know is there a solution for 8 hours.

Answer 1

Run the 3 hour twice alongside the 7 hour timer. You now know there's 1 hour left in the 7 hour timer. Start the 3 hour and when the 7 ends you flip the 3 back, giving 8 hours.