Methods for rapid addition and subtraction
If one of the terms to increase by several units, it should be obtained from the sum subtract the same number of units.
Example:
274 + 97 = 274 + (97 + 3) - 3 = 274 + 100 – 3 = 374 – 3 = 371
1996 + 759 = (1996 + 4) – 4 + 759 = 2000 + 759 – 4 = 2759 – 4 = 2755
If one of the terms increased by several units and the second at the same rate to reduce units, the amount is not changed. On the basis of the round-one term at the expense of another.
Example:
998 + 1526 = 1000 + 1524 = 2524
When subtracted, increased by several units, then, that the difference is not changed, it is necessary to increase and reduce the same amount of units.
Example:
5431 – 3996 = 5435 – 4000 = 1435
If delays reduced by several units, then to the resulting difference should be added the same number of units.
Example:
10013 – 9775 = 10000 - 9775 + 13 = 225 + 13 = 238
Methods for rapid multiplication and division
Multiplication by 9, 99, 999, etc.
Example:
167 · 9 = 1670 – 167 = 1503
26 · 99 = 2600 – 26 = 2574
Multiplication by a number close to unity any discharge.
Example:
615 · 98 = 615 · (100 – 2) = 615 · 100 – 615 · 2 = 61500 – 1210 = 60290
5015 · 1002 = 5015 · (1000 + 2) = 5015 · 1000 + 5015 · 2 = 5015000 + 10030 = 5025030
Multiply two-digit number to 11. To multiply two-digit number, the amount of which numbers less than 10, 11, it is necessary to write the digits of the sum of its digits.
Example:
63 · 11 = 693
54 · 11 = 594
To multiply by 11 two-digit number, the sum of digits is greater than or equal to 10, is necessary between the number of tens, increased by 1, and the number of units to write the difference between the sum of the digits of the number and the number 10.
Example:
86 · 11 = 946
Multiply by 5, 25, 125.
To multiply the numbers 5, 25, 125, it is sufficient to divide by 2, 4, 8, and multiplied by 10, 100, 1000 .
Example:
1246 · 5 = 6230, as 1246 : 2 = 623
6428 · 25 = 160700, as 6428 : 4 = 1607
8032 · 125 = 1004000, as 8032 : 8 = 1004
Division by 5, 25, 125. To divide the number 5, 25, 125, it is sufficient to multiply by 2, 4, 8 and divided by 10, 100, 1000 .
Example:
315 : 5 = 63, as 315 · 2 = 630
2025 : 25 = 81, as 2025 · 4 = 8100
10125 : 125 = 81, as 10125 · 8 = 81000
Good Luck!