You're guessing. Use the formula.
You START with 900. Why would there be only 900 still after 12 days?
Think about doubling.
12 days: 900*2 = 1800
24 days: 2*12 days = 900*2*2 = ??
Seriously, WRITE DOWN a working formulation. Don't try to do it in your head or on your calculator. Let the notation help you. Allow the algebra to soak into your brain and pour over your soul...
Ahem...Sorry, I got a little poetic, there.