Why can a variable be used to represent a number

Steve wants to develop a way to find the total cost of any item including sales tax. the sales tax in his city is 6%. his friend jenna writes the expression x + 6x where x is the cost of the item. explain whether her expression is correct based on the situation.

The table shows the balance of a money market account over time. write a function that represents the balance y(in dollars) after t years.

Which geometric principle is used to justify the construction below?