# Fast Arithmetic Tips

### Checking the result fast

- Divisibility by 5.

A number is divisible by 5 iff it ends with either 5 or 0. So you know right away that 123·345=42434 is wrong. It's a different question how to get the right answer (42435).

- Use divisibility criteria.

As above, divisibility criteria are useful in establishing whether a result is wrong. Note that this way you never may be sure whether the result is right. The following criteria is available:

- Divisibility by 3
- Divisibility by 9
- Divisibility by 7
- Divisibility by 11
- A number is divisible by 4 iff its 2-digit tail is divisible by 4. For example, 12345 is not divisible by 4 since 45 is not. However, 12348 is divisible by 4 since 48 is.
- A number is divisible by 8 iff its 3-digit tail is divisible by 8. For example, 12345 is not divisible by 8 since 345 is not. However, 12344 is divisible by 8 since 344 is. Indeed
344 = 320 + 24, where both addends are divisible by 8. - A number is divisible by 25 if it ends with one of the four: 00, 25, 50, 75. Therefore 234465 is not divisible by 25.
- The is a general way of deriving divisibility criteria for numbers such as 7, 11, 13, and other.

- Use rude bounds.

- 127·345 = 27455 is clearly wrong because
127 > 100 while345 > 300. Therefore, it must be that127·345 > 100·300 = 30000. - 2
^{20}= 524288 is clearly wrong because2 Actually^{20}= (2^{10})² = 1024² > 1000² = 1000000.524288 = 2 ^{19}. - A practical application.

- 127·345 = 27455 is clearly wrong because
- Use digital roots.

*Digital root*of an integer is, by definition, the sum of its digits computed recursively until only one digit remains. Thus to compute the digital root of 33572, we calculate first3 + 3 + 5 + 7 + 2 = 20. The digitl root of 33572 is then2 = 2 + 0. The important fact about digital roots is that

*the digital root of the product of integers equals the product of their digital roots*(downsized to a digital root if necessary.)348·11 = 3828. The digital roots of 348, 11, and 3828 are respectively 6, 2, and 3. Take the product of 6 and 2:

6·2 = 12 with the digital root equal to1 + 2 = 3, which is exactly the digital root of 3828.(Thanks are due to Jayesh Patel from Gujarat State (India) for reminding me of this useful property of the digital roots.)

The reason for this is that a digital root of a number is simply the remainder of the division of the number by 9. The operation of omputing digital roots is equivalent to the process known as "casting out 9s". Indeed, for any

[an error occurred while processing this directive]n ≥ 0; , 10^{n}≡1 (mod 9). |Contact| |Front page| |Contents| |Algebra| |Math magic|