900 dollars is her goal but not her "goal-goal" if you get what I mean. Since it takes her 2 weeks to earn 400 dollars, it'll take her a month to earn 800 dollars. if you add another 2 weeks, the time she gets paid, she'll have 1,200 dollars, more than enough to afford her camera. But in terms, it'll take her 33.5 days to earn 900 dollars.
My answer could always be wrong so don't take 100 percent of my word for it but to my knowledge, itll take her that long to earn 900 dollars.