K-4 MATHEMATICS STANDARDS

    WHOLE NUMBER COMPUTATION
    ACTIVITY I

    Standard's bullets addressed by activity

    • use a variety of mental computation and estimation techniques

    Technology

    • None at the beginning. Calculators can later be used to verify answers after problems are mentally computed.

    Description of Activity

  • Group or classroom management practices

      • Students work individually. They can also work in pairs or in small groups to share and discuss their strategies before sharing with the whole class.

    Teacher reads the following problem orally and instructs the students to figure out the answer in their heads­no pencil-and-paper computation:

      On my way to work this morning I stopped by the bakery to get something to eat for my usual on-the-run breakfast. I bought a half dozen ham and cheese croissants, one for myself and the rest to share with the office. The croissants were 99¢ apiece (no sales tax). How much did I pay for the croissants?

    After mentally calculating their answers, students share their strategies. Some of these may include:

    • I rounded each one off to a dollar, multiplied by 6 for the half dozen, and got $6.00. Because I added one penny for each croissant, I took off six cents from the total and got $5.94.
    • I multiplied 9 times 6 and got 54¢. Then I multiplied 90 times 6 and got $5.40. I added these together and got $5.94.
    • I just multiplied in my head: 9 x 6 = 54. Put the 4 down and carried the 5. Then I multiplied 9 times 6 again and got 54, plus the 5 equals 59. My answer is $5.94.

    There may be other strategies that the students invent. The purpose of this activity is to show that there are several ways to arrive at the same answer. Some strategies may be more efficient than others, but the important thing to remember is that the students are thinking. Also, by hearing more efficient strategies from others, they may try these strategies with other problems.

    Some questions the teacher might ask:

    • (to student with the first response above) Why did you round 99¢ to one dollar?
    • We now have three different ways and we all got the same answer. Does anyone else have a different strategy?
    • Was there a strategy different from yours that you thought was easier? Why do you think it was easier? Will you use that strategy?
    • How useful is mental math in the real world?
    • Have any of you used mental math while shopping for food, clothes, or toys? The teacher can also share how he/she uses mental math.
  • Extensions

    • Use mental math exercises several times a week, or on a daily basis. Use situations that are meaningful to the students. Students may even make up their own problems and give them to their classmates to solve.

    Students should also be encouraged to estimate their answers before computing for exact answers. Calculators can then be used to verify their answers and to see how close their estimates were.

    Teachers may also want to discuss situations in which mental math is most efficient to use, when calculators are most efficient to use, and when paper and pencil calculations are most appropriate.