Answer: 40%
Step-by-step explanation: The total number of pizzas is 12 + 18 = 30. Mitchel bought 12 of those 30 pizzas. To find the percentage you do the portion/total. Thus 12/30 = 0.4, as a proportion. To turn a proportion into a percentage simply multiply by 100. 0.4 * 100 = 40%