A $2\mathrm{.}5-$kg ball and a $5\mathrm{.}0-$kg ball have an elastic collision.$$ Before the collision, the $2\mathrm{.}5-$kg ball was at rest and the other ball had a speed of $3\mathrm{.}5$ m$/$s$.$ What is the kinetic energy of the $2\mathrm{.}5-$kg ball after the collision?$$

Hint: Elastic collision means kinetic energy is conserved. Because no external force is acting, momentum is also conserved.$$

You have to solve $2$ equations in $2$ unknowns

A$.$

$1\mathrm{.}7$ J

B$.$

$8\mathrm{.}1$ J

C$.$

$27$ J

D$.$

$14$ J