This month, your friend deposits $400 to save for a vacation. she plans to deposit 10% more each successive onth for the next 11 months. how much will she have saved after 12 deposits?

Neverminding for a second that she's depositing 10% of 400 every month

let's say, she goes to the bank, drops 400, and goes home and sits comfy for 12 months

now, the bank every month, is giving her 10%
how much is the amount at the end of the year? or at the 12th deposit?

well, is just a matter of compound interest