math tricks

Here are some math tricks I came across... good luck studying!

Times Four

Isn't it true that 4 is really 2 x 2? And since 2's are doubles, why not just double the number then double that answer? For instance, in the problem 4 x 6: 6 + 6 = 12 and 12 + 12 = 24.

Times Five

To quickly multiply by 5, divide the number in two and then multiply it by 10.  Often this can be done quickly in your head.

Example: 14 x 5 becomes 7 (half of 14) x 10 = 70

Remember any odd number x 5 will always end in a 5

Any even number x 5 will always end in a 0

Times Nine

When you need to multiply a single digit times 9, to figure the answer, start with "one less than" the number being multiplied (10's digit). The one's digit is the difference between this number and 9.

Examples:
9 X 4 = : One less than 4 is 3, plus 6 more make 9: 36. 9x4=36
9 X 5 = : One less than 5 is 4, plus 5 more make 9: 45. 9x5=45
9 x 6 =: One less than 6 is 5 -- 5 + ? = 9 . That's right -- 4! ... so 9 x 6 = 54
9 X 7 = : One less than 7 is 6, plus 3 more make 9: 63. 9x7= 63
9 X 8 = : One less than 8 is 7, plus 2 more make 9: 72. 9 x 7 = 72
 

Times Eleven

Did you know there are 2 patterns for the 11's? I think everyone knows how to arrive at the answers to the single digit numbers: 11 X 1, 11 X 2, etc. To multiply a single digit times eleven, just write the number down twice:

11 X 2 = 22
11 X 3 = 33
11 X 4 = 44
This only works through 11 X 9 = 99.
What about 11 X 11 and 11 X 12 and even more?
Here is a way to multiply 2 digit numbers times 11.

The Rule: When multiplying a 2 digit number times eleven, add the 2 digits together and put the sum between them - this is your answer.

Examples:
11 X 53: 5 + 3 = 8, so put the 8 between the 5 and 3. Your answer is 583!
11 X 12: 1 + 2 = 3; put the 3 between the 1 and 2. Your answer is 132.
11 X 42: 4 + 2 = 6; put the 6 between the 4 and 2. Your answer is 462.