To solve the problem, it is necessary the concepts related to the definition of area in a sphere, and the proportionality of the counts per second between the two distances.
The area with a certain radius and the number of counts per second is proportional to another with a greater or lesser radius, in other words,


M,m = Counts per second
Our radios are given by



Therefore replacing we have that,






Therefore the number of counts expect at a distance of 20 cm is 19.66cps
The energy absorbed by photon is 1.24 eV.
This is the perfect answer.
So that we do not contaminate it with microorganisms or garbage or other human stuff.
C, they didn't know any better