Wait, where did you get 23? I guess you were right using the summation operator though.

But we're talking about 28 days. o_O

27
Σ 2^i = 268435455
i=0

(2^28) - 1 = 286435455

My answer is too 268435455 (in penny)

I am just using a Mathematic formula, Geometric Series to solve this question. I learned that during my Computer Math lessons -.-

http://www.jimloy.com/algebra/gseries.htm
http://en.wikipedia.org/wiki/Geometric_series

We are using the formula S=a(1-r^n)/(1-r) as the value is infinite because r = 2 in this case (r = 2 because the value is double everytime, meaning common ratio r = 2) and a is refer to the 1st term of the series. Since your series goes like 1,2,4,8... which is a G.P (Geometric Progression)

Using sum of Geometric series will solve this,

a = 1
n = 28 (number of terms = number of days in this case)
r = 2 (because the value keep x2 after each term)

Therefore S, which is the sum of geometric series is

S = [1(1-2^28)]/(1-2)

which is

S = -268435455 / -1
   = 268435455

Hope that will solve your Mathematics problem. I don't use any excel nor written program to do this, just some simple mathematics formula

For further studies, you can read up on Arithmetic Series too, there's a formula for that too, Arithmetic series is for those additional cases, Arithmetic Progression (A.P), not multiplication case but for this case it's Multiplication case since the value get double up.

I already said that LONG ago
if you make it shorter and take out the redundant you get:
S = 1-2^28
Which is EXACTLY what I said on the first page.
Why did this thread need 4 pages of proving me right and sx wrong?
poor sx,,, stop picking on his math fail!
what would you know?!?
You failed at your own question
boo2u

Lmao, so many different ways to represent the same thing.

Basically all of the correct solutions involved something like this:

1 + 2 + 4 + 8 + 16 + 32 + 64 ...

Except for 2^28 - 1

And you're supposed to get 268435455 pennies.

Well I hoped it would end when I posted. Lol.