Ashley is riding a Ferris wheel that has a diameter of 40 metres. The wheel revolves at a rate of 1.5 revolutions per minute. Ashley's height above the ground, h, after t minutes can be given be modelled by the equation h = 21 - 20 cos (3πt)

In the four minutes that she spends riding the Ferris wheel, what is the total amount of time that Ashley's height is above 31 metres?