SOLUTION: Please help me solve this one. I guess this is a combination problem but I can't figure out my n and r. Thanks a lot! Seven identical balls are randomly placed in seven availabl

Question 48187: Please help me solve this one. I guess this is a combination problem but I can't figure out my n and r. Thanks a lot!
Seven identical balls are randomly placed in seven available containers in such a way that two balls are in one container. Of the remaining six containers, each receives at most one ball. Find the number of ways in which this can be accomplished.
Answer by stanbon(57222) (Show Source):
You can put this solution on YOUR website!
Seven identical balls are randomly placed in seven available containers in such a way that two balls are in one container. Of the remaining six containers, each receives at most one ball. Find the number of ways in which this can be accomplished.
1st: Pick a container to contain 2 balls: 7 ways
2nd: Select two balls to put in that container: 1 way as the balls are identical
3rd: Pick a container to be empty: 6 ways
4th: Put a ball in each of the remaining five containers: 1 way as the balls
are identical.
# of ways is 7*6 = 42
Cheers,
Stan H.