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## Homework Statement

A Helium 5 at rest decays into Helium 4 and a neutron.

Mass of Helium 4 = 6.648 x 10^-27

Mass of Neutron = 1.67493 x 10^-27

Helium 4 has a momentum of 1.903 x10^-20 and Ek 2.723x10^-14

## Homework Equations

E=mc^2

v= square root of (E2/m)

## The Attempt at a Solution

I have found the mass defect for both Helium 4 and the neutron. and Helium 5.

I think i can use E=mc^2, and then use E in v= square root of (E2/m). I am stuck in trying to figure out what I should use for m? The mass defect on the before or after the decay? Any help appreciated.