Having trouble with this Q

[amilss]

Hi. this is a revision question in which i keep getting the wrong answer for

"A mixture of butane gas (C4H10) and excess oxygen, which occupied 170ml at SLC was sparked. When the reaction ceased, the mixture was returned to SLC. The final volume of the gas mixture was 100ml. When this gas mixture was bubbled through concentrated NaOH solution, all the carbon dioxide was removed. The volume of gas remaining was 20ml. The equation for the combustion is:

2C4H10 (g) + 13O2 (g) ------> 8CO2 (g) + 10H2O (g)

What was the volume of butane in the original mixture?"

[totaled]

I THINK the initial information (final volume was 100ml) is only trying to put you off

if all the carbon dioxide was removed, and the volume of the gas decreased by 100ml-20ml = 80ml
then you know there was 80ml of CO2
n(CO2) = .08 / 24.5 = 0.00326530612
therefore, n(c) =n(CO2) = 0.00326530612
therefore, n(c4h10) = 1/4 x n(CO2) = 1/4 x 0.00326530612 = 0.00081632653

n = v/VM
therefore V = n x Vm
                 = 0.00081632653 x 24.5
                 = 0.02L = 20mL

is that answer correct?
I don't think the 170ml is necessary for the calculation, though i could be completely wrong :p
and also check my calculations, i don't have a calculator on me so i had to use google, i could've made a mistake copying and pasting here and there

[amilss]

Yes 20ml was the answer! Thank you very much