wiganworld   (Feedback, help, your ideas for new content.)

Here's Johnny's answer:

"To time 15 minutes exactly, start both sand timers running.

When the 7-minute timer has emptied, turn it over.

When the 11-minute timer has emptied, turn the 7-minute timer over again.

The 7-minute glass will have just run 4 minutes since you last turned it, so it will take 4 minutes to run back again - finish on 15 minutes exactly."