Solubility is the maximum amount of a substance that will dissolve in a given amount of solvent at a specific temperature. There are two direct factors that affect solubility: temperature and pressure. Temperature affects the solubility of both solids and gases, but pressure only affects the solubility of gases.
3.75 litres is the volume of the balloon indoors at a temperature of 25°C.
Explanation:
Data given:
initial temperature of the gas in balloon = -35°C or 238.15 K
initial volume = 3 litres
final temperature = 25 °C or 298.15 K
final volume =?
pressure remains constant
From the data given when pressure is constant Charles' law is applied.
= 
Rearranging the equation to know the final volume of the gas in balloon
V2 = 
V2 = 
V2 = 3.75 Litres
when the temperature of a gas is increased and pressure remains constant the volume of the gas increases.