Louise has 20 red pencils * 2 = <<20*2=40>>40 blue pencils.
She therefore has 20 red pencils + 40 blue pencils = <<20+40=60>>60 green pencils.
In total, she has 20 red pencils + 40 blue pencils + 60 green pencils + 40 yellow pencils = <<20+40+60+40=160>>160 pencils.
As each box holds 20 pencils, she needs 160 pencils / 20 pencils/box = <<160/20=8>>8 boxes.
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