Shoutbox

G16 should actually be "=G6-G15" or else you get the negative of it.  You could have checked this by actually making a graph and plotting the points to see that the y intercept had to be positive

EDIT: The third thing seems to be an interest formula except I don't think you did it right, you should add the last value, not the initial value and the interest should be fixed (or just write it in the form of a geometric progression....).