Let H be the event that head occurs and let T be the event that tail.
Then the complete Sample Space,S for the experiment given in the question is:
S={ HHH, HHT, HTH, THH, HTT, THT, TTH, TTT}
As we can see the total number of elements in the sample space is 8. Let us represent this by the letter N.
Therefore, N=8
Now, the event of interest to us is TTT and it's occurrence is once. Let represent this by the letter E.
Therefore, E=1
Thus, the probability of the event E occurring is given as:
[tex] P(E)=\frac{E}{N}=\frac{1}{8} [/tex]
[tex] \frac{1}{8} [/tex] is the required answer. This can be represented in percentage too as:
[tex] \frac{1}{8}\times 100=12.5 [/tex]%
