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:
Simplify by subtracting 30-12:
Subtract 18 from both sides:
Divide both sides by -4 (flip the sign when you multiply or divide by a negative):
You can only buy a whole number of gum packs, so:
Preston can buy up to 4 gum packs, assuming his money isn't stollen my monke