Calculating Acceleration of Rising Balloon

[iseidthat]

Homework Statement

What is the acceleration of a rising hot-air balloon if the ratio of the air density outside the balloon to that inside is 1.30? Neglect the mass of the balloon fabric and the basket.

Homework Equations

g=9.81 m/s^s

The Attempt at a Solution

I thought it would just be 1.3 times g=
1.3(9.81)=12.75 m/s

my homework says it is wrong...why?

 

[redargon]

if the ratio was 1:1 the acceleration would be 9.81m/s^2 downwards. So how much lighter is the air in the balloon actually?