How quick calculations

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: