Begin by expressing the relationship between the ovens with an equation. Use a variable to represent the number of burnt pizzas.
For every x pizzas burnt by Oven 2, Oven 1 burns 3x pizzas. It is given that 12 total pizzas are burnt, and the total number of burnt pizzas is a combination of pizzas burnt by Oven 1 and Oven 2:
x + 3x = 12
4x = 12
x = 3.
The variable x represents the number of pizzas burnt by Oven 2, so answer is 3 pizzas.