A body of mass 2kg moving with a speed of 4m s ^ - 1 makes an...

${p}_{i} = {p}_{f}$

$2 \times 4 = 2 \times 1 + {m}_{2} \times {v}_{2}$

${m}_{2}{v}_{2} = 6 \ldots ( 1 )$

By coefficient of restitution,
$1 = \dfrac{{v}_{2} - 1}{4} ⇒ {v}_{2} = 5m{s}^{- 1}$

by $( i )$

${m}_{2} \times 5 = 6$

${m}_{2} = 1 . 2kg$

${v}_{cm} = \dfrac{{m}_{1}{v}_{1} + {m}_{2}{v}_{2}}{{m}_{1} + {m}_{2}}$

${v}_{cm} = \dfrac{2 \times 1 + 1 . 2 \times 5}{2 + 1 . 2} = \dfrac{8}{3 . 2} = \dfrac{25}{10}$

$x = 25$