Answer
$P(E)=\frac{7}{8}$
Work Step by Step
H -> head
T -> tail
All the possible outcomes (sample space):
$S=[(H,H,H),(H,H,T),(H,T,H),(H,T,T),(T,H,H),(T,H,T),(T,T,H),(T,T,T)]$
We have that the total number of possible outcomes in the sample space is:
$N(S)=8$
We want at least one head (Event), that is, one head, two heads or three heads:
$E=[(H,H,H),(H,H,T),(H,T,H),(H,T,T),(T,H,H),(T,H,T),(T,T,H)]$
$N(E)=7$
$P(E)=\frac{N(E)}{N(S)}=\frac{7}{8}$
