17. A particle, $P$, is projected vertically upwards from the ground with a speed of 30 m s-1. A second particle, $Q$, is projected vertically upwards from the top of a tower, of height 25 m, with a speed of 10 m s-1 at the same time. Find the height above the ground, in metres and correct to 3 significant figures, of the particles when they are at the same height.