Asample of methane gas has a volume of 10 l at a pressure of 1.5 atm and a temperature of 20 °c. what volume does the gas occupy if the pressure is lowered to 0.9 atm at a temperature of 40 °c?

We can use combined gas laws to solve for the volume of the gas
 
where P - pressure, V - volume , T - temperature and k - constant

parameters for the first instance are on the left side and parameters for the second instance are on the right side of the equation 
T1 - temperature in Kelvin - 20 °C + 273 = 293 K
T2 - 40 °C + 273 = 313 K
substituting the values 

V = 17.8 L
volume of the gas is 17.8 L