The sum of a number and 12 is 20. what is the number?

--Answer--

The number is 8.
We verify:

x + 12 = 20 
x = 20 - 12
x = 8

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Build an equation from this phrase.

"The sum of a number and 12..." The word sum is the keyword of what operation we are doing. We are doing addition. When we continue the phrase we know we are adding a number (an unknown) and 12.

So, on the left-hand side we have x + 12.

"... is 20" In mathematics, the word 'is' can mean equal. So, the left-hand side will equal the number 20.

Now we have the equation x + 12 = 20.

Solve for x.

x + 12 = 20
x = 8 <-- Subtract each side by 12

So, x is equal to 8.