Question 881283
The question is unclear, but I'll make some assumptions.  Please resubmit if I've assumed incorrectly.
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Let's say there is a dress for sale that was originally $100.  20% off means the dress is now $80.  90% of that $80 is $72.
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Or, if the $100 dress has been discounted 90%, it's $10.  20% of that is .2 * 10 or $2.