SOLUTION: Dear math teacher, I am not sure why I am having a pretty hard time with combinations. I really nailed the Permutations section of the chapter but the combinations are not tha

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Question 475183: Dear math teacher,
I am not sure why I am having a pretty hard time with combinations. I really nailed the Permutations section of the chapter but the combinations are not that nice to me.
Here is the problem I am struggling with:
"A has 3 maps and B has 9 maps. Determine the number of ways in which they can exchange maps if each keeps his initial number of maps. "
Here is what I did: 12C3 + 12C9 = 220 + 220 = 440 but that is not the right answer. Thank you for your time and help.
Yours respectfully,
I.
Answer by scott8148(6628) (Show Source):
You can put this solution on YOUR website!
there are 3 ways that A can give B one map, and 9 ways that B can give A one map
___ 27 ways they can exchange ONE map ___ (3C1) * (9C1)

for TWO maps ___ (3C2) * (9C2)

for THREE maps ...

add the three results