Example of Working Backwards (Accounting for Events)

Heres an example of how you can account for events that happened in a Question.

John and Mary had some cookies at first. John gave 2/7 of his cookies to Mary. In return, Mary gave 1/4 of her cookies to John. In the end, John and Mary had the same number of cookies. What was the ratio of John's cookies to Mary's cookies at first?

Have fun.