Finding the maximum density of a balloon

[penzoate]

Homework Statement

A balloon is filled with helium at an atomospheric pressure. The skin of the balloon has a mass of 3.1 kg and the volume of the balloon is 18 m^3.

Homework Equations

What is the greatest weight of the balloon?

The Attempt at a Solution

In order to determine the maximum weight of the balloon, wouldn't you have to calculate the weight differences between the balloon and the air surrounding the balloon. Assuming this is the correct step,

18m^3*(density of air=1.293)*9.8-3.1*9.8=maximum weight
or
18m^3*(density of air=1.293+density of helium=.1786)-9.8*3.1=maximum weight
or
18m^3*(Density of air=1.293)*9.8-18m^3*(density of helium=.1786)*9.8-3.1*9.8=maximum weight

Which of these 3 calculations I attempted is correct

 

[berkeman]

You need to add the weight of the balloon skin, not subtract it. Other than that, your 3rd try looks correct to me.