Question

Keiko has seven colors of lanyard. She uses three different colors to make a keychain. How many different combinations can she choose?

Answer 1

Combination = doesn't matter what order
Permutation = order matters

There are two methods to work out combinations.

Method 1 List out possibilities
123 124 125 126 127 134 135 136 137 145 146 147 156 157 167
234 235 236 237 245 246 247 256 257 267
345 346 347 356 357 367
456 457 467
567

For a total of 35 combinations.

Method 2 Use a formula.
It's a rather complicated one, so only use it if you have a lot of possibilities.

$\frac{n!}{r!(n-r)!} = \frac{7!}{3!(7-3)!} = \frac{7!}{3!*4!} = \frac{7*6*5*4*3*2*1}{3*2*1*4*3*2*1} = \frac{7*6*5}{3*2*1}=\frac{210}6=35$

(n is the number of choices, r is the amount you choose, and ! is a function that multiplies together all numbers down to 1)