The question tells us he uses 4 6-inch pieces of tape to put up each poster (seems wasteful to me, but whatever). That's 24 inches of tape per poster, or 2 feet of tape per poster. Remember, the question asks for answers in feet. So if he puts up 5, as we're plugging in for n, he's using 5x2 feet of tape, or 10 feet. If he started with 300 feet, he'll be left with 290 feet.
Now, go to the answer choices, and see which one gives you 290 when you plug 5 in for n:
(B) 300 - 2(5) = 290
(D) 300 - (1/2)(5) = 297.5
(E) 300 - (1/4)(5) = 298.75
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