The temperature is presumably fixed, so we use Boyle's Law relating pressure and volume.
(a) The product of PV is constant so 30*10^5=V*2*10^5, 2V=30 so V=15cc. The pressure is doubled so the volume is halved.
(b) The pressure is multiplied by 5 so the volume is reduced by 5 to 30/5=6cc.