Question 21
A set $S$ is constructed as follows. To begin, $S=\{0,10\}.$ Repeatedly, as long as possible, if $x$ is an integer root of some polynomial $a_nx^n+a_{n-1}x^{n-1}$ $+\cdots+a_1x+a_0$ for some $n\ge1,$ all of whose coefficients $a_i$ are elements of $S,$ then $x$ is put into $S.$ When no more elements can be added to $S,$ how many elements does $S$ have?