Let n = number of gum packs he buys.
After he starts with 30, spends 12, then spends 4n, he must have at least 0 remaining. So:
[tex]30-12-4n\geq 0[/tex]
Simplify by subtracting 30-12:
[tex]18-4n\geq 0[/tex]
Subtract 18 from both sides:
[tex]-4n\geq -18[/tex]
Divide both sides by -4 (flip the sign when you multiply or divide by a negative):
[tex]n\leq 4.5[/tex]
You can only buy a whole number of gum packs, so:
[tex]n\leq 4[/tex]
Preston can buy up to 4 gum packs, assuming his money isn't stollen my monke
