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;
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Factors of 90 are;

Factors of 36 are;
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The highest common factor is;

Thus, the largest number of backpacks they can pack is 18.