Given,
mass of brick m = 1 kg
density of brick d = 2.5 g/cm3
= 2.5 * 1000 = 2500 kg/m3
So, volume of brick v = m/d
v =1/2500
v = 4 *10 -4 m3
when brick is immersed in water its displaces same volume of water as the volume of brick. Volume of water displace v = 4*10-4m3
m = 1000 *4 * 10-4
m = 4/10
m= 0.4 kg