Two steel sphere having radii a and 2a are released from height h. Collision is perfectly elastic. Velocity small sphere just after collision

Dear student

Let us consider m1 & m2 are masses of the sphere and velocity is v of the sphere before collision 'v' is the velocity of other sphere and fist sphere at rest after collision.
From conservation of energy 
1/2 m1v1^2 + 1/2 m2v2^2 = 1/2 m1v1^2 + 1/2 m2v2^2
1/2 m1v^2 + 1/2 m2v^2 = 1/2mv'^2 
v' = n(m1-m2)/m2
m1m2 +m2^2 =m1^2+m2^2 -2m1m2 
m2 = m1/3 
The volume of the fiist sphere is v and the volume of other is v/3
Now, the radius of the sphere is r and other sphere is r′

4/2 *pi*r'^3 = v/3 
r'= r/3*(1/3)
Good Luck