Because Bryce started off with $450 then his a1 should be $450. Then, the amount of money he will have after n months is, an = 13(an-1) where an is the amount of money he has and an-1 is the amount of money he has in his account from the month prior.
I think that Harry should use 1.5-2 cups of flour for his cake. It depends on the size of the cake so if its pretty small, 1.5 cups should work. If you want to take a risk, use more cups of flour.