# Physics (Energy and Momentum)

posted by
**Andrew** on
.

A body of 5.10 kg mass makes an elastic collision with another body at rest and continues to move in the original direction, but with one-fourth of its original speed. What is the mass (in kilograms) of the struck body?

Assuming a perfectly elastic collision, KEi=KEf

ive tried solving this many ways and i keep ending up with 2 equations and 3 unknowns.