We are going to use one of our pressure laws, specifically Boyle's Law, for this one: P1V1 = P2V2
A fun mnemonic I like to use to remember this equation is Purple Violets are Purple Violets --> PV =PV
0.79 atm (20 L) = (1.50 atm) (x)
x = (0.79 atm (20 L)) / (1.50 atm)
x = 10.53 L
So at 1.50 atm, the volume occupied is 10.53 L.