It's a 100-foot fence, and Ben whitewashed 10 feet, so there are 100 - 10 = <<100-10=90>>90 feet left after Ben is done.
Billy whitewashed a fifth of that, or 90 / 5 = <<90/5=18>>18 feet.
There is now 90 - 18 = <<90-18=72>>72 feet of fence left.
Johnny whitewashes a third of that, or 72 / 3 = <<72/3=24>>24 feet.
That leaves 72 - 24 = <<72-24=48>>48 feet of fence left to be whitewashed.
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