Q.4 letters are to be posted in 4 post boxes. If any number of letters can be posted in any of the 4 post boxes, what is the probability that each box contains only one letter?

For a random experiment whose outcomes are all equally likely (no outcome is preferred over any other), the classical definition assigns to an event $A$ the probability $P(A)=n(A)/n(S)$, the fraction of all outcomes that are favourable to $A$. This single ratio - favourable outcomes over total outcomes - is the computational engine of the whole chapter: once a sample space is built and the outcomes favouring an ev …