Question

A child of mass 40.0 kg is swinging on a swing with ropes of length 4.0 m. How long does it take the child to swing from the maximum height on one side to the other?

Answer 1

Half of the period of the swing (which is essentially a pendulum).
It will be independent of the mass.

The period of a pendulum is
2 pi sqrt(L/g)
Half of that will be your answer.
Having two ropes instead of one does not change the formula from that of a simple pendulum.

Answer 2

Thanks. I was confused by the way the question was worded and my answer was 2 times bigger than what the answer is supposed to be.