Stan needs to write 2,000 words because 5 x 400 = <<5*400=2000>>2,000
This will take him 40 minutes because 2,000 / 50 = <<2000/50=40>>40
He drinks 1/4 an ounce per minute because 15 / 60 = 1/4
He will drink 10 ounces of water while writing the paper because 40 x (1/4) = <<40*(1/4)=10>>10
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