You may want to reference (pages 209 - 211) section 5.4 while completing this problem. part a a weather balloon is inflated to a volume of 26.9 l at a pressure of 730. mmhg and a temperature of 25.9 ∘c. the balloon rises in the atmosphere to an altitude where the pressure is 370. mmhg and the temperature is -13.6 ∘c. assuming the balloon can freely expand, calculate the volume of the balloon at this altitude.

For this question we can used the combined gas law equation
for a fixed amount of gas PV/T = constant 

where P - pressure, V-volume and T - temperature 
parameters for the first instance are given on the left side and parameters for the second instance are given on the right side of the equation.
Temperature should be in Kelvin
temperature in K = temperature in celcius + 273
T1 = 25.9 °C + 273 = 298.9 K
T2 = -13.6 °C + 273 =259.4 K
substituting the values into the equation , V - volume at the altitude can be found out 

V = 46.1 L