For the same reasons as the last problem...
s(n)=20(1-(1/2)^n)/(1-1/2)
s(n)=40(1-(1/2)^n)
But this time we are not starting at one so we must subtract the sum of s(2) from the sum of s(12) so:
40((1-(1/2)^12-(1-(1/2)^2)
40(0.249755859375)
≈9.99 (to nearest one-thousandth)
