Math tricks can be a source for quick calculations. Here are some tricks to try to boost your math confidence.
Multiplying by 6
If you multiply 6 by an even number, the answer will end with the same digit. The number in the ten's place will be half of the number in the one's place.
Example: 6 x 4 = 24.
The 11 Rule
This is a quick way to multiply two-digit numbers by 11 in your head.
Separate the two digits in your mind. Add the two digits together. Place the number from Step 2 between the two digits. If the number from Step 2 is greater than 9, put the one's digit in the space and carry the ten's digit. Examples: 72 x 11 = 792.
57 x 11 = 5 _ 7, but 5 + 7 = 12, so put 2 in the space and add the 1 to the 5 to get 627
Multiply Large Numbers in Your Head
To easily multiply two double-digit numbers, use their distance from 100 to simplify the math:
Subtract each number from 100. Add these values together. 100 minus this number is the first part of the answer. Multiply the digits from Step 1 to get the second part of the answer.
Divisibility Rules
Divisible by 2 if the last digit is a multiple of 2 (210). Divisible by 3 if the sum of the digits is divisible by 3 (522 because the digits add up to 9, which is divisible by 3). Divisible by 4 if the last two digits are divisible by 4 (2540 because 40 is divisible by 4). Divisible by 5 if the last digit is 0 or 5 (9905). Divisible by 6 if it passes the rules for both 2 and 3 (408). Divisible by 9 if the sum of the digits is divisible by 9 (6390 since 6 + 3 + 9 + 0 = 18, which is divisible by 9). Divisible by 10 if the number ends in a 0 (8910). Divisible by 12 if the rules for divisibility by 3 and 4 apply. Example: The 210 slices of pizza may be evenly distributed into groups of 2, 3, 5, 6, 10.
Multiplication with 9 Using Fingers
A simple way to do the "9" multiplication table is to place both hands in front of you with fingers and thumbs extended. To multiply 9 by a number, fold down that number finger, counting from the left.
Examples: To multiply 9 by 5, fold down the fifth finger from the left. Count the fingers on the left of the folded down finger as tens, and count the fingers on the right of the folded down finger as ones. In this case of 9 x 5, the answer is 4 tens on the left and 5 ones on the right, which equals 45.
To multiply 9 times 6, fold down the sixth finger, giving an answer of 54.
