Annexes in math may sound complex but they are in fact very simple. However, the word "annex" has multiple meanings, which can make it confusing. Annexing a number to either side of an equation may involve either adding or multiplying. Annexing may be useful when attempting to solve algebra.
Annexing by Addition

If you start off with the equation:
2x + 6 = 4y + 16
You may add a number to either side of the equation. For example, you may add 4 to either side:
2x + 10 = 4y + 20
Here annexing simply means adding.
Annexing by Mulitplication

If you start off with the equation:
44,670 x 5 = 223,350
You can multiply either side of they equation by annexing zero:
446,700 x 5 = 2,233,500
In this case annexing means multiplication.
Purpose of Annexing by Addition

Annexing a number to either side of the equation makes it possible to complete the equation. For example:
2x + 10 = 4y + 20
Rearranging gives you:
2x  4y = 20 10 = 10
Purpose of Annexing by Multiplication

If you are asked to make the following calculation:
44,670 x 5 =
You may find it easier if you multiply either side of the equation by annexing 0:
(44,670 x 10) / 2 = 446,700 / 2 = 223,350
This method is helpful if you find it easier to divide by 2 than to multiply by 5. In many cases this will be true, and annexation can therefore be a useful technique.
References
 "Complete Mathematics"; T. Johnson et al.; 2010
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