640/10=64.
What's the question?
And figuring that there will be 10 winners is funny math based on them selling 10 times the normal amount of tickets as there aren't normally 172 million tickets sold, which is the number required for there to be an average of one guess per possible combination.
They would have to sell 1.72 billion tickets for there to be an average of 10 guesses on each possible combo. That's nearly 6 tickets for every American alive, including those in states where they don't sell MM tickets.