Answer
The total kinetic energy of the neutron and the pion is $40~MeV$ which is $6.4\times 10^{-12}~J$
Work Step by Step
We can find the mass $M$ that is lost:
$1115~MeV/c^2 = 940~MeV/c^2+135~MeV/c^2+M$
$M = 1115~MeV/c^2 - 940~MeV/c^2-135~MeV/c^2$
$M = 40~MeV/c^2$
We can assume that the missing mass is converted into kinetic energy. We can find this energy:
$E = Mc^2$
$E = (40~MeV/c^2)(c^2)$
$E = 40~MeV$
$E = (40~MeV)(\frac{1.60\times 10^{-13}~J}{1~MeV})$
$E = 6.4\times 10^{-12}~J$
The total kinetic energy of the neutron and the pion is $40~MeV$ which is $6.4\times 10^{-12}~J$.
