Just that really 
My 15 week old has been off her milk for a few weeks now and I've been struggling to get her to feed, however over the past couple of days due to being out and about I've given her some of the ready made formula in a carton (aptamil first) and the has drained the lot!
I don't know if it's just coincidence because she has been really hungry or if she actually prefer's the ready made stuff
Any one else experienced this?