12 is 30% of what number

30% * n = 12 → you know that : 30% = 30/100

(30/100) * n = 12 → you know that : 30/100 = 3/10 = 0.3

0.3 * n = 12 → then divide by 0.3 on each side

0.3 * n/0.3 = 12/0.3 → you simplify

n = 40 of the number 40