A 20 liter sample of gas is kept under a constant pressure of 100 kpa. the temperature is lowered from 360 k to 90 k. what is the new volume

To solve this we assume that the gas inside the balloon is an ideal gas. Then, we can use the ideal gas equation which is expressed as PV = nRT. At a constant pressure and number of moles of the gas the ratio T/V is equal to some constant. At another set of condition of temperature, the constant is still the same. Calculations are as follows:

T1 / V1 = T2 / V2

V2 = T2 x V1 / T1

V2 = 90 x 20 / 360

V2 = 5 L