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In one semester, Doug and Laura spent a ...

In one semester, Doug and Laura spent a combined 250 hours in the tutoring lab. If Doug spent 40 more hours in the lab than Laura did, how many hours did Laura spend in the lab?

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The correct answer is 105. Let D be the number of hours Doug spent in the tutoring lab, and let L be the number of hours Laura spent in the tutoring lab. Since Doug and Laura spent a combined total of 250 hours in the tutoring lab, the equation D + L = 250 holds. The number of hours Doug spent in the lab is 40 more than the number of hours Laura spent in the lab, and so the equation D = L + 40 holds. Substituting L + 40 for D in D + L = 250 gives (L + 40) + L = 250, or 40 + 2L = 250. Solving this equation gives L = 105. Therefore, Laura spent 105 hours in the tutoring lab.
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