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Let x be the number of hours the plumber worked.
The total cost for the plumber's work is 70 + 50x = $285.
Subtracting 70 from both sides, we get 50x = 215.
Dividing both sides by 50, we find that the plumber worked x = 215/50 = <<215/50=4.3>>4.3 hours. Answer: \boxed{4.3}.