A pendulum oscillates $40$ times in $4$ seconds.

 Find its time period and frequency:

Given,

The number of oscillations, $\mathrm{n} = 40$

Time taken,$\mathrm{t} = 4 \mathrm{s}$
Time period , 

$\mathrm{T} =\frac{\mathrm{Time} \mathrm{taken}, \mathrm{t}}{\mathrm{Number} \mathrm{of} \mathrm{oscillations}, \mathrm{n}}$

Substituting the values,

$\mathrm{T} =\frac{4}{40} = 0.1 \mathrm{s}$

Now,
$\mathrm{Frequency}, \mathrm{f} = \frac{\mathrm{Number} \mathrm{of} \mathrm{oscillations}, \mathrm{n} }{ \mathrm{Time} \mathrm{taken}, \mathrm{t}}$
Substituting the values,

$\mathrm{Frequency}, \mathrm{f} = \frac{40}{ 4} = 10 \mathrm{Hz}$

Thus time period,  $\mathrm{T} =0.1 \mathrm{s}$ and frequency, $\mathrm{f} = 10 \mathrm{Hz}$.