If James bought 400 fish, the number of fish that were bad is 20/100*400 = <<20/100*400=80>>80
The remaining fish that he used to make the sushi rolls is 400-80 = <<400-80=320>>320
If 40 fish are required to make one sushi roll, 320 fish will make 320/40 = <<320/40=8>>8 sushi rolls
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