Lyle was a careful craftsman, but he wasn't very
creative. For example, he only sold two types of
wooden benches: stained and unstained. The un-
stained benches cost $20 each, and the stained
cost $32 each. If Lyle sold 21 benches last month
(stained and unstained combined) and collected a
total of $600, how many unstained benches did he
sell?