Have you had no instruction on Permutations and Combinations?
How many different 4-number combinations can be made from a set of 20 numbers?
1-2-3-4, 1-2-3-6, etc.
Identical problem: There are 20 different CDs on sale.
. . . . . . . . . . . . . You have enough money to buy four of them.
. . . . . . . . . . . . . In how many ways can you select four CDs?
Since you are choosing the CDs and tossing them in a shopping bag,
. . the order of the CDs is not important.
Hence, this is a "combinations" problem.
The Combination Formula says: . ways.
The derivation goes like this:
You have 20 options for your first choice,
. . . .and 19 options for your second choice,
. . . .and 18 options for your third choice,
. . . .and 17 options for your fourth choice.
It seems that you have: . possible choices.
But this long list includes choices like: and
. . Since the order is not important, these two selections are identical.
Since 4 objects can be arranged in different orders,
. . our number is too large by a factor of 24.
. . Our list has 24 times as many choices as it should have.
Hence, the corrected answer is: .