Hey there! I'm happy to help!
When dealing with picking things and finding the number of possibilities, we can be dealing with two different types of things: permutations or combinations. We use permutations when order is important to the groups and we use combinations if order is not important.
It appears here that order is not important here as the subcommittee doesn't have to be in a specific order, so we will be using combinations.
So, we have ten seventh graders to choose from. Then we will have 9, and then 8. So, we multiply these numbers of choices.
10×9×8=720
Now, we need to divide by 6. This is to show that order doesn't matter. Why? Because there are 6 possible ways to organize 3 seats on the committee (3×2×1), and we want to divide out all of the different orders so it is just the specific group of people and not their different organizations that are counted.
720/6=120
Now, we need to find this with the eighth graders.
8×7×6=336
And we divide by the number of seats factorial (3 seats for the eighth graders, so 3×2×1=6 again)
336/6=56
Now, we multiply our numbers for seventh and eighth graders to find all off the possibilities.
120×56=6720
So, there are 6720 subcommittees possible.
Have a wonderful day and keep on learning! :D
