Replace b with 4: 100a+200(4)=4000. 100a+800=4000. Combine like terms: 100a=3,200. Get the unknown by itself: A=32. She can download 32 old games if she downloads 4 new ones. Same steps: 100a+200(5)=4000. 100a+1000=4000. 100a=3000. A=30. She can download 30 old games if she downloads 5 new ones. :)