What are dollar bills made of?
Dollar bills are made of a type of special cloth made from recycled materials, and other top-secret materials not made known to the public.
They were made during the Great Depression when Franklin Delano Roosevelt was president. During the depression, the only way to rebuild America was to lend people money. This is when dollar bills were made, called Federal Reserve Notes. This was they way that put the Great Depression to an end. (check a dollar bill and you'll see federal reserve note on the top)
Reading the question very carefully, I'd have to say "none". If the stack consists of "900 dollar bills", then there are no "50 dollar bills" in it. But the questioner is probably referring to equivalent values. 900 dollar bills have a face value of $900.00 . In order to match that same face value with $50.00 bills, you need 18 of them.