Question 1058494
Let x represent the unknown number.


Three times that number is then represented as 3x.


Since this value must be "greater than or equal to 12, and less than 21", 
you can write:


12 &#8804; 3x < 21


Dividing through by 3 leaves:


12 / 3 &#8804; 3x / 3 < 21 / 3


4 &#8804; x < 7


Solution: The number must be greater than or equal to 4, and less than 7.


In interval notation, the solution is [4, 7).