it goes like this..

A kid wants to collect an album with stickers. The album has n-different stickers. To get a sticker, the kid has to buy a Chocholate bar which has a randomly packed sticker with it, and it costs 15 $ . How many $$ does he has to spend to collect all the stickers in the album ?

All i know is ( i suppose ) to use (n+k-1)!/k!(n-1)! to find all the combinations of the packed stickers in the chocolate bars ( duplicates included ) ... after that im lost

please help !!

thanks in advance