Can someone help illustrate how the solution of "36" is reached in this problem? I got as far as figuring out that the sum of all the "Before" units = X and the sum of all the "After" units = X+36 but we did nothing but flounder around spasmodically after that. Any help appreciated. Thanks!

Lisa made some kiwi tarts, apple tarts and peach tarts in the ratio 2:3:5. After making another 12 tarts each, the ratio becomes 7:9:13. Find the number of apple tarts she has now.