Lesson Plan # 19 - Combinations

  1. (5 min) Mental Math
    1. Start with 26, double it, then add 9 [61]
    2. What is 30% of 70? [21]
    3. What is 35% of 200? [70]
    4. What is 20% of 110? [22]
    5. How many ways can you reorder the letters in the word "CAT"? [6]

  2. Follow the handout on "combinations", where order doesn't count.
  3. Choose 5 students' names and then write out all possible 2-person groups. There are only 10 of them, because order doesn't count.
  4. Discuss the "handshake" problem. Draw a box like this to show that this problem is similar to picking 2 letters out of a word. The number of people in the group goes in the first box, then after you choose one person to handshake with another, the number of people available to handshake with him or her goes in the second box.
Key idea: For 2-person groups,
when order doesn't count,
you divide the number of permutations by 2
  • (Remainder of class) In-class Exercise.

  • Hand out homework as students successfully complete the in-class exercise.