Two solid rubber balls A and B  having masses 200 and 400 gm respectively are moving in opposite directions with velocity of A equal to 0.3 m/s. After collision the two balls come to rest, then the velocity of B is

Initial linear momentum of system =  ${\mathrm{m}}_{\mathrm{A}}{\overrightarrow{\mathrm{v}}}_{\mathrm{A}}+{\mathrm{m}}_{\mathrm{B}}{\overrightarrow{\mathrm{v}}}_{\mathrm{B}}$
                                        = 0.2 × 0.3 + 0.4 × v_B
Finally both balls come to rest 
$\therefore$ final linear momentum = 0 
By the law of conservation of linear momentum 
      0.2 × 0.3 + 0.4 × v_B = 0
$\therefore {\mathrm{v}}_{\mathrm{B}}=-\frac{0.2\times 0.3}{0.4}=-0.15\mathrm{m}\text{}/\text{}\mathrm{s}$