### Instruction: If the ratio of $b$ to $a$ is 3, then what is the value of $a$ when $b=12-5a$?### Response: Let's set up our ratios. We know that $\frac{b}{a}=3$.
We also know that $b=12-5a$. So, we can substitute that value for $b$ in the first ratio.
That gives us $\frac{12-5a}{a}=3$.
If we multiply both sides by $a$, we get $12-5a=3a$.
Adding $5a$ to both sides, we get $12=8a$.