Home > Mental calculation

1 Calculating differences: a - b

1.1 Direct calculation

When the digits of b are all smaller than the digits of a, the calculation can be done digit by digit. For example, evaluate 872 - 41 simply by subtracting 1 from 2 in the units' place, and 4 from 7 in the tens' place: 831.

1.2 Indirect calculation

When the above situation does not apply, the problem can sometimes be modified:

• If only one digit in b is larger than its corresponding digit in a, diminish the offending digit in b until it is equal to its corresponding digit in a. Then subtract further the amount b was diminished by from a. For example, to calculate 872 - 92, turn the problem into 872 - 72 = 800. Then subtract 20 from 800: 780.

• If more than one digit in b is larger than its corresponding digit in a, it may be easier to find how much must be added to a to get b. For example, to calculate 8192 - 732, we can add 8 to 732 (resulting in 740), then add 60 (to get 800), then 200 (for 1000). Next, add 192 to arrive at 1192, and, finally, add 7000 to get 8192. Our final answer is 7460.

2 Calculating products: a × b

2.1 Multiplying by 10

To multiply a number by 10, simply add an extra 0 to the end of the number.

2.2 Multiplying by 2

In this case, the product can be essentially calculated digit by digit. This is not exactly the case because it is possible to have remainder, but if there is a remainder, it is always 1, which simplifies things greatly. Still, the product must be calculated from right to left: 2 × 167 is by 4 with a remainder, then a 2 (so 3) with another remainder, then a 2 (so 3). Thus, we get 334.

2.3 Multiplying by 5

To multiply by 5, first multiply by 10, then divide by 2. Adjoin a 0 to the right end of the number. Then read the number from left to right, dividing the digits by 2, and eventually adding 5 to the next digit if the digit that was divided was odd (after having been divided). For example, 176 × 5= 1760 ÷ 2. Digit by digit we get 0 (in the thousands digit), 5 + 3, 5 + 3, and 0. This gives 880.

2.4 Multiplying by 9

Note that 9 = 10 - 1. Thus, to multiply by 9, multiply the number by 10 and then subtract the original number from this result. For example, 9 × 27 = 270 - 27 = 243.

2.5 Using hands to multiply numbers

This technique allows a number from 6 to 10 to be multiplied by another number from 6 to 10.

This method uses the fingers of both hands, face to face:

Here are two examples:

• 9 × 6

above:

below:

- 5 fingers below make 5 tens - 4 fingers above to the right - 1 finger above to the left

the result: 9 × 6 = 50 + 4 × 1 = 54

• 6 × 8

above:

below:

- 4 fingers below make 4 tens - 2 fingers above to the right - 4 fingers above to the left

result: 6 × 8 = 40 + 2 × 4 = 48

How it works: each finger represents a number (between 6 and 10). Join the fingers representing the numbers you wish to multiply (x and y). The fingers below give the number of tens, that is (x - 5) + (y - 5). The digits to the upper left give (10 - x) and those to the upper right give (10 - y), leading to [(x - 5) + (y - 5)] × 10 + (10 -x) × (10 - y) = x × y.

2.6 Multiplying a two-digit number by 11

Add the two digits together, write down the answer, then append the tens' digit on the left and the units' digit on the right. Thus, for example, 17 × 11 = 187, 35 × 11 = 385.