a sample of gas has a volume of 400 mL when measured at 25 degrees C and 760 torr. What volume will it occupy at 25 degrees C and 195 torr?

Yes
it is Boyle's law
P1V1=P2V2
  From initial experimental condition pick up P1 and V1 -400 mL when measured at 25 degrees C and 760 torr.
From final experimental condition Pick up P2 -25 degrees C and 195 torr
Calculate V2
Check P1 and P2 should be in same units