Let, the number of weeks, required for buying that camera = x
She earns = $400 per week
Then in x weeks, she will earn = 400x
Now, 400x = 900
x = 900 / 400
x = 9/4
In short, She needs 2 1/4 weeks or if she gets paid after a whole week, then 3 weeks [ 'cause you'd have round-it off to the nearest greater value ]
Hope this helps!