I've finally decided that i'll buy into some iCloud storage. I can get 10 extra gigs for $20. I only have a 16gb phone so the entire phone can be backed up even when it's capacity is filled. My question is that could i play another 10 gigs for $20 on top of the 15 gigs i had for 25 gigs or would i have to buy 20 gigs for $40.