Let 'x' be the number of books that cost $4 and let 'y' be the number of books that cost $2. Since Courtney purchased 87 books, then we have the following equation:
[tex]x+y=87[/tex]also, she spent a total of $200, then, we have the next equation:
[tex]4x+2y=200[/tex]we want to know how many $4 books Courtney bought. Then, in the first equation, we can solve for y to get the following:
[tex]y=87-x[/tex]if we substitute this equation on the second equation, we get:
[tex]\begin{gathered} 4x+2(87-x)=200 \\ \Rightarrow4x+174-2x=200 \\ \Rightarrow4x-2x=200-174 \\ \Rightarrow2x=26 \\ \Rightarrow x=\frac{26}{2}=13 \\ x=13 \end{gathered}[/tex]therefore, Courtney purchased 13 books that cost $4