9 students volunteer for a committee. How many different 2-person committees can be chosen?

There can only be 4 2 person committees one person will have to leave

We can start this problem by giving each student a number from 1 through 9, and then matching each student up with one other student of a different number. The easiest way to do this is to make a chart with the numbers 1 through 9 along the top and the numbers 1 through 9 along the side. The next step should be to go through the boxes and label if the two students can work together. This means that they haven't been in a group with each other before, and that they aren't in a group with themselves. An easy way to do this is to look at a multiplication chart, and when you do this, you should reach an answer of around 36 different groups.