1. The problem statement, all variables and given/known data 2. Relevant equations Principle of conservation of momentum, Σpi=Σpf 3. The attempt at a solution Since the nucleus is stationary, initial momentum is zero. Unified atomic mass constant, a.m.u = 1.66 x 10^-27 Mass of the new nucleus: (A-4) x a.m.u Mass of proton: 4 x a.m.u 0= ((A-4) x a.m.u )(u) + (4 x a.m.u )(-v) 4v = (A-4) u v= (A/4 -1) u So I chose A, But the answer is B, where have I gone wrong?