Jasper bought two bags of cookies. There were 4 vanilla cookies and 6 chocolate chip cookies in the first bag, and there were 5 vanilla cookies and 5 chocolate chip cookies in the second bag. Jasper closed his eyes and randomly picked one of the bags, and then grabbed a cookie from that bag without looking inside or seeing which bag he took the cookie from. It turns out that Jasper grabbed a chocolate chip cookie. Given that Jasper grabbed a chocolate chip cookie, what is the probability that he picked it from the first bag of cookies