The largest number of backpacks they can pack is the highest common factor of the number of pencils, note pads and granola bars divided among the backpacks.
Number of pencils = 144;
Number of note pads = 90;
Number of granola bars = 36.
Factors of 144 are;
Factors of 90 are;
Factors of 36 are;
The highest common factor is;
Thus, the largest number of backpacks they can pack is 18.