A minute is 60 seconds, so 6 minutes is 6 * 60 = <<6*60=360>>360 seconds.
A hand-crank sharpener can sharpen 360 / 45 = <<360/45=8>>8 pencils in 6 minutes.
An electric sharpener can sharpen 360 / 20 = <<360/20=18>>18 pencils in 6 minutes.
Therefore, an electric sharpener can sharpen 18 - 8 = <<18-8=10>>10 more pencils than a hand-crank in 6 minutes.
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