Dual Nature Of Light Question 180

Question: A neutron moving through container filled with stationary deuterons. The neutron successively collides elastically and head on with stationery deuterons one at a time. The mass of the neutron is equal to half that of the deuteron. How many such collision would be required to slow the neutron down from 81 eV to 1 eV. [Neglect the relativistic effect]

Solution:

Answer: 2.00