Question

Graphite (carbon with atomic mass of 12 atomic mass units (amu)) has some desirable properties as a neutron moderator in a nuclear reactor. The neutron (approximately 1 amu) needs to be slowed in order for the reaction to proceed. Assuming each collision is elastic, how many times must the neutron collide with a carbon atom (at rest) until its final speed is less than 25% of its initial speed?