The designers of a park are creating a water slide and have sketched a preliminary drawing. The length of the stairs is 30 feet, and its angle of elevation is $45^{\circ} .$ (See figure.)
(a) Find the height $h$ of the slide.
(b) Find the angle of depression $\theta$ from the top of the slide to the end of the slide at the ground in terms of the horizontal distance $d$ a rider travels.
(c) The designers want the angle of depression of the slide to be at least $25^{\circ}$ and at most $30^{\circ} .$ Find an interval for how far a rider travels horizontally.