To enter an exponent, positive or negative, use the EE (Enter Exponent) key (the shifted function of the , key just above the 7 key). Do 5 2ND [EE] (-) 1 0 / 5 2ND [EE] (-) 1 0 ENTER and you'll see 1.
What I think you're doing (I can't be sure without seeing your keystrokes) is calculating
5*10^-10/5*10^-10 which is interpreted as (5*10^-10/5)*10^-10, which indeed is 10^-20.
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