Solving Systems of Linear Equations using Substitution

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. $$b+y=21$$ 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. $$\{\begin{array}{l}b+y=21\\ b=2.5y\end{array}$$