Half a box

This is the lid of a box.  You can use a smaller piece of paper to make the other half and use this lid to cover the other half... or you can make that the lid...

Two weeks ago I had a phone interview but failed it miserably.  I couldn't answer things like whats the type of cout(it's an ostream).  The interviewer told me I could answer the hard problems but not the easy one... it was more of I could answer the big ideas(like whats a semaphore, so easy) but couldn't answer specifics of a language.  I learned a lot from it though.  I thought my knowledge of c++ was fine, nope it's terrible.
So anyway fun puzzle for you all from the interview
There are 10 coin making machines and 1 of them is broken.  They're all suppose to be making coins that weigh 10g but one of the machines is always making coins that weigh 9g.  You have a digital scale, how should you find which one is the broken machine.
hint: it takes only 1 measurement(this isn't really a hint more of a requirement) and it's not guessing and hoping you're right...
It took me a couple minutes...