Mental Math

In this segment, Luna, the friendly frog, can add up 9 calendar dates in her head faster then Noah, who is using a calculator. Your students will be amazed to see that Luna is faster every time. How does she do that?  Luna will teach Noah (and your class) how to get the sum of the 9 dates so quickly – and with some practice, soon all of your students will be able to perform this amazing math trick!

How it Works

Luna, that sneaky frog, instead of adding the 9 dates, took the middle number in the square and multiplied it by 9…

Voila! (Works every time!)

The math level is for grades 4-6.

(Students should be able to do
multiplications of 9 in their head)

Math Concepts

In this episode, students use Addition, Multiplication and Mental Math.

1.

Let’s take this calendar page (August 2015) as an example. The red frame is covering 9 dates.

2.

Let A (in our example, #10) be the central number of the 3×3 square.

3.

Now, look at the date in the center. The day before is A-1, the day after is A+1

4.

The date above (exactly a week earlier) is A-7 and the date below is A+7

5.

As you can notice, the pairs have opposite numbers that cancel each other out

6.

…and the same thing happens here, with the last pair.

7.

All these pairs add up to 8A, and with the central number add up to 9A.

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