Explanation:

There are 56 ways to choose 2 girls out of 8 and there are 7 ways to choose 1 boy out of 7.

Because we use the word "and", we multiply 56 by 7, and we get 392.

But then, there's the possibility that we choose 1 girl and 2 boys.

There are 8 ways to choose 1 girl out of 8 and there are 42 ways to choose 2 boys out of 7.

Again, we multiply due to the word "and", so we get 336

We can add 392 and 336 because we choose 2 girls and 1 boy OR 1 girl and 2 boys.

Finally, we get an answer of 728.

Solution #2:

You can choose 1 girl and 1 boy first.

There are 8 ways to choose 1 girl and 7 ways to choose 1 boy.

Now there are 13 students left and one more student needed to help.

There are 13 ways to choose 1 more student. Gender doesn't matter now as we already have 1 girl and 1 boy.

Now we multiply as we need to choose 1 girl, 1 boy, AND another student.

Once again, we get an answer of 728.