Tell your students you will beat them adding up 5 x 5 digit numbers in your head when they are using calculators.
Method:
1. Ask a student to write down 2 x 5-digit numbers on the board.
2. You rapidly write a 5-digit number underneath.
3. Ask another student to write another 5-digit number.
4. You write another 5-digit number quickly.
5. You have 5 by 5 digit numbers. Say ‘Go’. You instantly write down the answer.
Stand back.
This is how it works:
N1 = 97413
N2 = 28619
N3 = 71380 (Each digit in N3 that you write down must add up to 9 with digits in No. above)
N4 = 64231
N5 = 35768 (Once again each digit in N5 must add up to 9 with digits above)
Now you will instantly write down the sum of these five numbers as
297411
Da! DA!
The trick is to subtract 2 from N1 and put it in front:
N1 = 97413
N1 -2 = 97411
Sum of 5 numbers = 297411
This is why it works:
Hint: When you get your students to add up the five 5-digit numbers on a calculator you will beat them, but they will also get many different answers as a number of students will key incorrect numbers.
Requirements: A Pack of Cards and a willing assistant.
Take 10 cards from pack including the 10 Diamonds.
Lay out card in 3 columns in a pattern of 4 down, 2 down, 4 down.
Put the 10 Diamonds at the top of the left hand column.
Group students around. You turn your head away and ask a student to point to a card.
You carry on, think allowed and then ask your assistant to point to cards asking:
Is it this one?
No.
Is it this one?
No.…………………………………………………………………………………
When your assistant points to the 10 Diamonds he will show you the position using the diamonds. The cards are laid out on the table in the pattern of the diamonds on the 10 Diamonds.
Whenever your assistant asks is it this one? You will know.
Write the word carrot inside your math text book. Do not show students.
When Mathspig was a mathspiglet we used to play this trick. It doesn’t work on everyone, but it works often enough.
Ask a student to say 15 times 15 fifteen times.
Then ask them to name a vegetable. Students say carrot 90% (I’m guessing) of the time.
To make this trick more dramatic send 10 students out of the class before you begin and tell the other half what you are about to do. Students return one at a time. How often do they say ‘carrot’? What % of students say carrot?
