Who decided to put the dollar sign after the number?

Dollar 20 implies one dollar and twenty cents anyways

It isn’t meant to represent speech. The symbol tells you the meaning of what follows. I don’t know why it’s done that way—we have $10.00, but 10¢—but that is the way it’s done.

I don’t understand your comment about one dollar and twenty cents. (And isn’t your question backward? We place the dollar sign before the number.)