Concept:
The number of left cosets of a subgroup \(H\) in a group \(G\) is called the index of \(H\) in \(G\).
It is given by
\[
[G:H]=\frac{O(G)}{O(H)}
\]
Step 1: Find the order of \(H\).
Given,
\[
H=\{1,15\}
\]
So the number of elements in \(H\) is
\[
O(H)=2
\]
Step 2: Write the order of \(G\).
The group \(G\) has order
\[
O(G)=8
\]
Step 3: Find the number of left cosets.
\[
[G:H]=\frac{O(G)}{O(H)}
\]
\[
[G:H]=\frac{8}{2}
\]
\[
[G:H]=4
\]
Step 4: Final answer.
\[
\boxed{4}
\]