Brittney can chop 15/5 = <<15/5=3>>3 onions in one minute.
Carl can chop 20/5 = <<20/5=4>>4 onions in one minute.
Brittney can chop 3 x 30 = <<3*30=90>>90 onions in 30 minutes.
Carl can chop 4 x 30 = <<4*30=120>>120 onions in 30 minutes.
Carl can chop 120 - 90 = <<120-90=30>>30 more onions than Brittney in 30 minutes.
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