Mental Math with Tricks and Shortcuts
SUBTRACTION
Techniques:
1) Learn to calculate from left to right: 1427 – 698 = (800 – 100) + (30 – 10) + 9 = 729
2) Think in terms of what number added to the smaller equals the larger: 785 – 342 = 443 (left to right)
3) Add a number to the larger to round to next highest 0; then add same number to the smaller and subtract:
496 – 279 = (496 + 4) – (279 + 4) = 500 – 283 = 217 (left to right)
4) Add a number to the smaller to round to the next highest 10, 100, 1000 and etc.; then subtract and add
the same number to the result to get the answer: 721 – 587 = 721 – (587 + 13) = (721 – 600) + 13 = 134
5) Subtract a number from each number and then subtract: 829 – 534 = 795 – 500 = 295
DIVISION
Techniques: Examples
Divide by parts of divisor one at a time: 1344/24 = (1344/6)/4 = 224/4 = 56
Method of Short Division 340 Remainders (3, 4, and 0 during calculations) 7)1792
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256 ßAnswer
Divide both divisor and dividend by same number to get a short division problem
10
972/27 divide both by 9 = 3)108
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36
Dividing by 5, 50, 500, and etc.: Multiply by 2 and then divide by 10, 100, 1000, and etc.
365/5 = 730/10 = 73
Dividing by 25, 250, 2500, and etc.: Multiply by 4 and divide by 100, 1000, 10000, and etc.
Dividing by 125: Multiply by 8 and then divide by 1000
36125/125 = 289000/1000 = 289
It can be divided evenly by:
2 if the number ends in 0, 2, 4, 6, and 8
3 if the sum of the digits in the number is divisible by 3
4 if the number ends in 00 or a 2 digit number divisible by 4
5 if the number ends in 0 or 5
6 if the number is even and the sum of the digits is divisible by 3
7 sorry, you must just try this one
8 if the last three digits are 000 or divisible by 8
9 if the sum of the digits are divisible by 9
10 if the number ends in 0
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