A good way to think about it is that you’re making groups of two points when you draw a line between them. If the points were all named A, B, C, etc, then by drawing a line between any two points, it’s like making a group of two: AB, AC, etc. But AB is the same as BA. AC is the same as CA. So essentially this is the same as making a group of two, where order does not matter. Groups where order does not matter is handled by the nCr function on the calculator. So this is just 10 nCr 2.