Answer
$40\times 10^3\;Hz$.
Work Step by Step
Period $T=25\;\mu s$.
$T=25\times 10^{-6}s$
The relation between frequency and the period is
$f=\frac{1}{T}$
Plug the value of $T$.
$f=\frac{1}{25\times 10^{-6}s}$
We can write $\frac{1}{s}=Hz$.
$f=\frac{1}{25\times 10^{-6}}Hz$
Simplify.
$f=0.04\times 10^6\;Hz$
$f=40\times 10^3\;Hz$.
