Boyle's law says Pressure and volume are inversely related. One goes up and other goes down (vice versa).
P1 V1 = P2 V2
(2.5) (1) = (0.4) (x)
x= 6.25 L
The answer should make sense because your pressure went from high to low which means your volume should opposite, from low to high, and that's what we see.
