Acylinder, with a piston pressing down with a constant pressure, is filled with 1.90 moles of a gas (n1), and its volume is 42.0 l (v1). if 0.600 mole of gas leak out, and the pressure and temperature remain the same, what is the final volume of the gas inside the cylinder?

Avagadro's law gives the relationship between volume of gas and amount of moles of gas. It states that at constant temperature and pressure, volume of gas (V) is directly proportional to number of moles of gas (n).
V/n = k
where k - constant 

V1 = 42.0 L
n1 = 1.90 mol
n2 = 1.90 mol - 0.600 = 1.30 mol
substituting the values in the equation

V = 28.7 L
Volume of the gas is 28.7 L 

The answer is c. 320 g