Let m1=100 g=0.1 kg be the mass of first ball, m2=200 g=0.2 kg be the mass of second ball.
u1 initial velocity of first ball before collision =3 ms−1
u2 initial velocity of second ball before collision =0 ms−1
v1 final velocity of first ball after collision =1.5 ms−1
v2 final velocity of second ball after collision
Applying law of conservation of momentum we can write,
m1u1+m2u2=m1v1+m2v2
Substituting values
0.1×3+0.2×0=0.1×1.5+0.2×v2
0.3=0.15+0.2×v2
Or v2=0.75 ms−1