To solve the problem, we need to use the given function f(x) = 4 + z(0.25) where x is the amount of 25% orange juice drink added and z is the amount of 100% orange juice drink added.
We know that initially 4 gallons of pure juice are present. So, the total amount of juice in the mixture is 4 + x gallons.
When we add 2 gallons of 25% orange juice drink, x = 2 and z = 0. So, using the function f(x), we get:
f(2) = 4 + z(0.25)
= 4 + 0(0.25)
= 4
Therefore, the concentration of orange juice in the drink after adding 2 gallons of 25% orange juice drink is 4%, expressed as a percent but without the percent sign.
Answer: 4