Determine the fraction of kinetic energy lost by a neutron (m1 = 1.01 u) when it collides

Question:

Determine the fraction of kinetic energy lost by a neutron (m1 = 1.01 u) when it collides head-on and elastically with a target particle at rest which is
(a) 11H (m = 1.01u);
(b) 21H (heavy hydrogen, m = 2.01 u);
(c) 126C (m = 12.00 u);
(d) 20882Pb (lead, m = 208 u).