Answer:
12
Explanation:
20x^2 + 160 = 400 + 220x
20x^2 - 220x - 240 = 0
20(x^2 - 11x - 12) = 0
20(x - 12)(x + 1) = 0
x - 12 = 0; x = 12
x + 1 = 0; x = - 1. (can't be negative)
So answer is 12 hours
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