A girl with a mass of 40 kg is swinging from a rope with a length of 2.5 m. What is the frequency of her swinging?

For a pendulum with a massless rope 'L' meters long, swinging through

a small arc, the period of the swing is

2 π √ (L/g) seconds.

and it doesn't depend on the mass of the thing on the end of the rope ...

that could be a pebble or a bus.

For the girl on the 2.5-m rope,

Period = (2 π) √ (2.5 / 9.8) = 3.17 seconds

Frequency = 1 / period = about 0.315 Hz.