jmc

First we begin by solving the system of equations $$\begin{array}{rl}2a+4b& =5,\\ 3b& =a.\end{array}$$Making the substitution for $a$ from the second equation to the first, we get $2(3b)+4b=5$, which simplifies to $10b=5$. Solving for $b$, we find that $b=\frac{1}{2}$. Plugging this into the second equation above, we obtain $a=3\cdot \frac{1}{2}$. Hence $3a=3\cdot \frac{3}{2}=\boxed{\frac{9}{2}}$.