The number of permutations of n objects, taken r at a time, is:The number of combinations of n objects, taken r at a time, is:A group of 8 seniors can be combined into how many different subgroups of 6? The answer is 28:
A group of 4 non-seniors can be combined into how many different subgroups of 2? The answer is 6:
Any one of the 28 subgroups of seniors can be combined with any one of the 6 subgroups of non-
seniors, so the total number of combinations of the seniors and non-seniors is 28 × 6 = 168.
The answer is (C).
If you’re good at memorizing formulas—if you can master the permutations and combinations
formulas without too much trouble—go ahead and do it. But all these formulas probably won’t get
you more than one right answer on the Math 2 test, so they don’t deserve a whole lot of time and
effort.