Since the exterior angle measures have the ratio $3:4:5$, they are $3x:4x:5x$ for some value of $x$. Each exterior angle is supplementary to an interior angle, so the measures of the interior angles of the triangles are $180^\circ - 3x$, $180^\circ - 4x$, and $180^\circ - 5x$. The sum of the interior angles of a triangle equals $180^\circ$, so we have
\[(180^\circ -3x) + (180^\circ - 4x) + (180^\circ - 5x) = 180^\circ.\]Simplifying this equation gives $-12x = -360^\circ$, so $x = 30^\circ$. Therefore, the smallest interior angle has measure $180^\circ - 5x = \boxed{30^\circ}$.
Multiply first equation by -4 and the second by 2 so you can drop off the a values then add the equations together. Then solve for b. You should get b=6.
Then plug b into one of the original equations and solve for a and you should get 10.75
So a plate of spaghetti is $10.75 and a salad is $6.
