Ordering

• Permutations ordered, combinations unordered

• To determine the number of ways we can flip a coin n times and get k heads, we use the formula for unordered combinations

• Strange because it looks like we are ordering

• two heads and one tail (thh, hth, hht)

• What was combined going from balls to coins?

• second head, third head: thh
• third head, second head: thh