Hello there,

at first I would like to tell that it´s quite hard for me to write this in English but I have no other choice. I got this example from my math teacher because she accused me and my friends of cheating during the test. Everyone got own math problem which we have to solve - then she will delete the bad mark. I would ask in some math forum in my country but I´m afraid she could see it. She made up these math problems specially for us - so there are no similar math problems on the Internet. She would know that it is from me. I spend a lot of time trying to calculate it but I´m completely lost.

The example: There is 20 math problems and in every (of those 20) math problems you can make 3 mistakes and count it in wrong (different) way. There are 3 girls and the girls "accidentally" (they cheated) made the same 6 mistakes in those 20 math problems. What is the probability?

So that's it. I hope someone here could help me. I don't even know if I should count it like - 60 possible mistakes, from this I choose 6 mistakes.. So 60nCr6? I don't think so.. And what with those 3 girls? Where to multiply it?

If you can count it for me, please tell me even the process how you get there. I would really like to understand it.

Thank you!