Agroup of six tourists arrive at an airport gate 15 minutes before flight time, but only two seats are available. how many different groups of two can get on the plane? how many different groups of two cannot get on the plane?

You need to calculate how many 2 person combinations can be made from 6.
The formula is n! / [r! * (n-r)!]
6! / [2! * 4!] =
6 * 5 * 4! / 2! =
15
The answer to the second part of the question is also 15.
Source:
http://www.1728.org/combinat.htm