We have seen the sample space and the random experiment of throwing a fair die. The sample space of this experiment would be S = {1, 2, 3, 4, 5, 6}.
We want to find the probability of the event A = a number less than 4 turns up
The outcomes which are favorable to A are {1, 2, 3, 4}
These outcomes are equally likely. That is, they have equal chances of happening.
So, according to classical definition of probability,
P(A)=number of events f avorables to Atotal number of outcomes
=46
⇒30P(A)=30×46
=20