Multiplication
Tips for Multiplication
1. If you are multiplying 5 times an even number: halve the number you are multiplying by and place a zero after the number.
Example:
5 x 4, half of 4 is 2, add a zero for an answer of 20.
Multiplying 5 times an odd number: subtract one from the number you are multiplying, then halve that number and place a 5 after the resulting number.
Example: 5 x 5: -1 from 5 is 4, half of 4 is 2, place a 5 at the end of the resulting number to produce the number 25.
Another example: 5 x 3: -1 from 3 is 2, half of 2 is 1, place a 5 at the end of this number to produce 15.
2.You likely all know the 10 rule (to multiply by 10, just add a 0 behind the number) but do you know the 11 rule? It is as easy! You should be able to do this one in you head for any two digit number. Practice it on paper first!
To multiply any two digit number by 11:
For this example we will use 54. Separate the two digits in you mind (5__4). Notice the hole between them! Add the 5 and the 4 together (5+4=9). Put the resulting 9 in the hole 594. That's it! 11 x 54 = 594. The only thing tricky to remember is that if the result of the addition is greater than 9, you only put the "ones" digit in the hole and carry the "tens" digit from the addition. For example 11 x 57 ... 5__7 ... 5 + 7=12 ... put the 2 in the hole and add the 1 from the 12 to the 5 in to get 6 for a result of 627 ... 11 x 57 = 627Practice it on paper first!
3. To square a number that ends in 5, multiply the tens digit by (itself+1), then append 25. For example: to calculate 25 ?25, first do 2 ?3 = 6, then append 25 to this result; the answer is 625. Other examples: 55 x 55; 5 ?6 = 30, answer is 3025. You can also square three digit numbers this way, by starting with the the first two digits: 995 x 995; 99 ?100 = 9900, answer is 990025.
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