statistics math help

Question

When testing for current in a cable with

nine

​color-coded wires, the author used a meter to test

two

wires at a time. How many different tests are required for every possible pairing of

two

​wires?

Answer 1

There are 9 ways to choose the first wire; 8 ways to choose the second. We don't care about order (for example Blue/Orange is the same test as Orange/Blue), so we have to divide by two to account for this double counting.

The result is 8*9/2. The same result is given by the combination function: C(9,2)