The Problem with the Hourglass

Some students have to take a test in 9 minutes. Due to an electromagnetic pulse, every watch has stopped working and we only have two hourglasses to measure time.

These two hourglasses (the only ones we have in the high-school) last for 7’ the first one and 4’ the second one. 

How can we measure exactly 9 minutes (neither one second longer, nor shorter) using these hourglasses?

Tip: try to start both hourglasses at the same time. What happens after exactly 4 minutes?

Deadline: October the 25th.

Printable version available.

And we continue with the 90's...

Oasis - "Wonderwall"