A 2-column table with 4 rows. Column 1 is labeled Gallons with entries 2, 4, b, 10. Column 2 is labeled miles with entries 56, a, 168, c. Alexa is planning to drive across the United States in her car. She started making a table to represent the proportional relationship between gallons of gas and miles driven. The table shows how far she can travel for each gallon of gas that she uses.

What is the constant of proportionality?
miles per gallon