SAT Mathematics Practice Test 2

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 $\mathit{D}+\mathit{L}=250$ 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 $\mathit{D}=\mathit{L}+40$ holds. Substituting $\mathit{L}+40$ for D in $\mathit{D}+\mathit{L}=250$ gives $\left(\mathit{L}+40\right)+\mathit{L}=250$, or $40+2\mathit{L}=250$. Solving this equation gives $\mathit{L}=105$. Therefore, Laura spent 4 hours in the tutoring lab.