Without the discount, each pair cost $10 so 3 pairs cost 3*$10 = $<<10*3=30>>30
With a discount of 10% on the total cost, she can pay only $30-($30*(10/100)) = $<<30-(30*(10/100))=27>>27
By buying 3 at once in order to get the discount, she can save $30-$27 = $3
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