Answer
$10$ possible wavelengths of light emitted
Work Step by Step
If an electron starts at level $n$, the number of possible different wavelengths (transitions) is:
Number of wavelengths
$=\frac{n(n-1)}{2}$
In our case $n=5$, so we find
$\frac{5(5-1)}{2}=10$
There are $10$ possible wavelengths of light emitted.
