Let b be the amount of blue paint he uses in the mix and y be the amount of yellow paint. Since he needs to make a total of 21 gallons of green paint, we can write the first equation as follows.
We also know that he must use 2.5 times more blue paint than yellow paint in the mix. We can write this as b=2.5y. If we combine the equations, we have the following system of equations.
To find how much blue and how much yellow paint he uses in the mix, we have to solve the system
for b and y. Since b is already isolated in Equation (II), we will substitute 2.5y for b in Equation (I) and solve the resulting equation for y.