The independent variable is the variable that is being manipulated or controlled in an experiment, and it is typically plotted on the x-axis of a graph. In this equation, "c" represents the number of thank you cards being purchased, which is something that the student can control or manipulate. The student can decide to buy 1 card, 2 cards, 3 cards, etc., and the cost will vary accordingly.
The dependent variable, on the other hand, is the variable that is being measured or observed, and it is typically plotted on the y-axis of a graph. In this equation, "t" represents the total cost of the thank you cards, which is something that depends on the number of cards being purchased. The cost is not something that the student can control directly, but rather it is determined by the number of cards being purchased.
Therefore, in the equation t = 2c, "c" is the independent variable because it is being manipulated by the student, and "t" is the dependent variable because it depends on the value of "c".