Based on the given ratio of 2:1 for blue to green paint, we can set up a proportion to determine the amount of blue paint needed.
Let x represent the number of gallons of blue paint needed.
Therefore, the amount of green paint needed would be (1/2)x gallons.
Since the total amount of paint needed is 6 gallons, we can set up the following equation:
x + (1/2)x = 6
To combine the like terms, we can multiply 1/2 by x, resulting in:
(3/2)x = 6
To isolate x, we can divide both sides of the equation by 3/2:
(3/2)x / (3/2) = 6 / (3/2)
Simplifying, we get:
x = 4
Thus, Angelina should buy 4 gallons of blue paint.