problem: The ratio of two numbers is $3:5$. When 4 is subtracted from the lesser of the two numbers and 8 is added to the larger of the two, the new ratio is $2:7$. What is the greater of the two numbers before 8 is added to it?
solution: Let $a$ be the smaller of the two numbers and let $b$ be the larger of the two. Then $\dfrac{a}{b}=\dfrac{3}{5}$, so $5a=3b$. Additionally, $\dfrac{a-4}{b+8}=\dfrac{2}{7}$, so cross multiplying gives $7(a-4)=2(b+8)$. We now have a system of two linear equations; solving gives $a=12$, $b=20$. Since the question asks us for the value of $b$, our answer is $\boxed{20}$.