Before a factory can ship a part it is inspected twice There

Before a factory can ship a part, it is inspected twice. There are 7 machines available for testing and each part can be tested on any 2 of the machines. How many ways are there to select the 2 machines that the part will be tested on?

Solution

There are 7 machines available for testing and each part can be tested on any 2 of the machines. How many ways are there to select the 2 machines that the part will be tested on?

Select 2 machines from 7.

7C2 = 21

21 ways