Answer:
To get the probability of tossing fewer than three heads, just add the probability if X is equal to 0, 1 and 2.
[tex]p_{X < 3} =p_{X=0}+p_{X=1}+p_{X=2}=\frac{1}{8}+\frac{3}{8}+\frac{3}{8}=\frac{7}{8}[/tex]
Alternatively, just subtract the probability if X = 3 to 1.
[tex]p_{X < 3} =1-p_{X=3}=1-\frac{1}{8}=\frac{7}{8}[/tex]
