If three times a number equals 21, what is the value of the number?

To determine the value of the number in the equation "three times a number equals 21," you can set up the equation as follows:

Let the unknown number be represented by ( x ). According to the problem, this translates mathematically to:

[ 3x = 21 ]

To isolate ( x ) and find its value, divide both sides of the equation by 3:

[ x = \frac{21}{3} ]

Simplifying the right side gives:

[ x = 7 ]

Thus, the value of the number is 7. This matches with the choice labeled as C. Understanding how to manipulate equations is essential here. The equation's setup reflects how direct multiplication and division operate, and solving for ( x ) demonstrates how to reverse the operation applied to the unknown number.