To solve such problems we need to know about Additive Inverse.
The additive inverse of a number is the opposite of the number, therefore, the same number that is at the opposite side of the number line.
For example, the additive inverse of number 54 is -54, similarly, the opposite of the number 6 is -6.
The sum of (5x+20) and (the opposite of 20),
[tex]5x+20+(-20)\\= 5x+20-20\\=5x[/tex]
Therefore the sum of (5x+20) and (the opposite of 20) is 5x.
As we can see above that on solving the expression above the only term that is left is 5x. therefore, to make the expression equivalent. we need to equate the expression with 5x.
[tex]5x+20-20=5x\\[/tex].
Learn more about Additive Inverse:
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