Math Trick #1
1. Pick a number, but know that you have to add and subtract, multiply
and divide, so a smaller number may be a bit easier. (limit them to a
number 1-10 maybe?)
2. Add one to the number
3. Multiply the number by 3
4. Subtract your number
5. Add five
6. Divide by two
7. Subtract your secret number
8. Map the digit you got to a letter in the alphabet
1=A 2=B 3=C 4=D 5=E ….
9. Think of a country in Europe that starts with that letter (there is a
list in the power point).
10. Take the second letter in that country’s name and think of an animal
that starts with that letter.
11. Now think of the color of that animal.
Are you thinking of a grey elephant from Denmark?
How does it work?
Pick a number:
Add one:
Multiply by 3:
Subtract your number:
Add 5:
Divide by 2:
Subtract your number:
say x
x+1
2x + 3
x+4
4
3x + 3
2x + 8
The Magic Number 8:
Write down your phone number (7 digits only).
1 3 5 7 9 1 1
Write down 8 after your number
13579118
Multiply your phone number by 8 and write it down after the 8:
1357911x8=10863288
135791181 0 8 6 3 2 8 8
Now add up the single digits of your number. If the sum is more than one digit, take that
sum and add up its digits. Continue adding up digits until only one digit is left.
:
1 + 3 + 5 + 7 + 9 + 1 + 1 + 8 + 1 + 0 + 8 + 6 + 3 + 2 + 8 + 8 = 71
now for the 71, add up single digits: 7+1 = 8
General MacArthur's Favorite Number Game
Visitors, especially young ones, often waited for General Douglas MacArthur to
perform a miracle, or at least utter something profound, when they met him. The
General could not always live up to his press notices.
So, MacArthur started using a mathematical trick that he described as "the old
number 115" as a way to maintain his image, have some fun, and mystify his guests.
Ask students to write down the number of month in which he/she was born.
(January is 1, February is 2, and so on).
Then have that person write down:
Month of birth (Example February): 2
Double it: 4
Add 5: 9
Multiply by 50: 450
Add in students age (Example 40): 490
Subtract 365: 125
At this point, ask a few students to call out the total, which in this case, is 125.
In your head, secretly add 115.
In this example, you will end up with 240.
Then proclaim, "Why, you were born in February and you're 40 years old."
Here's how you did it. The first number -- and the second if the total is a fourdigit number -- will indicate your friend's birth month, once you've added good old
115. The last two digits will indicate his age.
This trick will work no matter what the age or month of birth because you have
good old 115 on your side. At least, it always did for MacArthur.
How does it work?
Let m = month and a = age.
Then following the instructions above:
Month of birth: m
Double it: 2m
Add 5: 2m + 5
Multiply by 50: 100m + 250
Add friend's age: 100m + 250 + a
Subtract 365: 100m - 115 + a
Add 115 to get: 100m + a. (The 100 just moves the month over 2 places)