how do you find all numbers that make 12 subtracted from three times a number and less than 21 true?

Let the numbers be represented by n. Then

 

3n - 12 < 21

 

Add 12 to both sides:

3n < 33

 

Divide both sides by 3:

n < 11

 

So any number less than 11 satisfies the condition.

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