Concept:
When a ratio and a total are known, individual quantities can be found.
Step 1: Analyze Statement (I).
Total students:
\[
90.
\]
No information regarding distribution between boys and girls.
Statement (I) is insufficient.
Step 2: Analyze Statement (II).
Ratio:
\[
7:8.
\]
Total number is unknown.
Possible values:
\[
7,8
\]
or
\[
14,16
\]
or
\[
21,24.
\]
Statement (II) is insufficient.
Step 3: Combine both statements.
Ratio sum:
\[
7+8=15.
\]
One part:
\[
\frac{90}{15}=6.
\]
Girls:
\[
8\times 6=48.
\]
Hence the number of girls is uniquely determined.
Therefore both statements together are sufficient.