PIMSO 2022 - Science - Grade 9 - National Round Question 17

An engineer is attempting to have an idea of the roughness of a piece of wood by measuring its coefficient of friction. The table and the accompanying graph below show his measurements of the minimum force needed to push objects of different weights.
Normal (N): 6, 12, 18, 24, 30. Force applied (N): 2, 7, 11, 13, 18.
Given the data, what is the coefficient of friction that the engineer should report?