How do you find the "whole" when you know a part and its percentage — e.g. 30 is 15% of what?

Divide the part by the percentage-as-a-decimal (or scale up from 1%).

30 is 15%, so 1% is 30 / 15 = 2, and 100% is 2 × 100 = 200.

Check: 15% of 200 = 30. ✓ The general move: whole = part ÷ (percent/100). Finding 1% first (part ÷ percent) and multiplying by 100 keeps the arithmetic small and avoids decimal-point slips.