In the context of the graph showing Kellen's pencils left each week, the independent quantity typically represents the input variable, which is often time in weeks, while the dependent quantity is the output variable, which in this case is the number of pencils.
The points you listed are (1, 15), (2, 12), and (3, 8), where the first number in each pair represents the week (independent quantity), and the second number represents the number of pencils left (dependent quantity).
Therefore, the number that identifies one of the independent quantities is 3 (the week number).