15. Let $a$ and $b$ be relatively prime positive integers and let $n$ be a positive integer. We call a solution $x, y$ of the linear diophantine equation $a x+b y=n$ nonnegative when both $x$ and $y$ are nonnegative.
a) Show that whenever $n \geqslant(a-1)(b-1)$ there is a nonnegative solution of this equation.
b) Show that if $n=a b-a-b$, then there are no nonnegative solutions.
c) Show that there are exactly $(a-1)(b-1) / 2$ positive integers $n$ such that the equation has a nonnegative solution.
d) The post office in a small Maine town is left with stamps of only two values. They discover that there are exactly 33 postage amounts that cannot be made up using these stamps, including $46 c$. What are the values of the remaining stamps?