Our Math Tricks
We studied "math tricks" in our Everyday Mathematics book.
Subtraction Surprise

To start: Tell a friend that you have become really fast at addition. To prove it, you will add five 3-digit in your head in a couple seconds.

Materials: calculator

1. Tell your friend to write a 3-digit number on a scrap of paper.
Every number must not be the same.

2. Tell your friend to repeat step 1. The same rules apply.

3. Last number. This is the number you notice of thiers.

4. Now comes your part. Write a number so when you add up thier first number and yours it equals 999.

5. Write one more so that the second number of thiers and your second number equals 999 again.

6 Wait a bit. Then give the sum of the 3 numbers they wrote down and the two you did.

7. Why does this trick work?

Look at our trials:

 Example Number one:   693   349   809   306 +650 2807 Example Number two:   867   204   140   132 +795 2138 Example Number three:   912   786   987   087 +213 2985

## Materials: calculator

1. Tell your friend to secretly write a 3-digit number on some paper. Again, each digit must be different.

2. Tell your friend to write the number backwards underneath the first number.

3. Have the friend use the calculator to subtract the lesser number from the greater number.

4. Then say "Tell me the digit in the hundreds place or the digit in the ones place. You also have to tell me if it is in the ones or hundreds place.

5. Wait a bit then give the answer.

Check out these trials:

 Example Number one 934 439     934 -439  495 Example Number two:   567   765     765 -567  198 Example Number three:   809   908     908 -809   099

To start: Tell your friend that you are sooooo good at adding that you can tell what the problem is just by looking at the answer.

Materials: calculator and a calendar

1. Tell your friend to chose a month and to secretly circle, highlight, or whatever you want to do to any three date that are next to each other in a row or a column.

2. Now tell your friend to add all three dates together. Tell him/her to give you the calculator showing the sum on the screen.

3. Ask if the dates circled are in the row or column.

4. Wait a bit then give the numbers in the problem.

Trials:

 SUNDAY MONDAY TUESDAY WEDNES. THURSDAY FRIDAY SATURDAY 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

Orange: "column"=45          8, 15, 22

Blue: "row"=54                    17, 18, 19