When two bodies collide elastically, then:

A block $A$ of mass $m$ moving with the velocity $\text{'}v\text{'}$ along a frictionless horizontal track and a block $B$ of mass $\frac{m}{2}$ moving with velocity $2 v$ collides elastically. The final speed of the block $A$ is:

A particle of mass $m$ moves with velocity $v_0=20 \mathrm{m} {\sec }^{-1}$ towards a wall that is moving with velocity $v=5 \mathrm{m} {\sec }^{-1}.$ If the particle collides with the wall elastically, the speed of the particle just after the collision is:

STATEMENT-1: In an elastic collision between two bodies, the relative speed of the bodies after collision is equal to the relative speed before the collision.

because

STATEMENT-2: In an elastic collision, the linear momentum of the system is conserved