Subtracting 10x from the given equation gives
120 = 15x
Then dividing by 15 gives
8 = x
Zorah will have to play 8 hours in order to break even (cover her expenses exactly).
If there are 272.5 rotations in a mile, and 5280 ft in a mile, then there are 5280 ft in 272.5 rotations. Divide 5280 by 272.5 gives us 19.4 ft/rotation.
That's the circumference of the wheel, and since c = πd, 19.4/π = d, d = 6.2 ft. The diameter of the wheel is how tall it is.
Does this help? Any questions?
Divide through everything by <em>b</em> :

Since <em>a/b</em> < <em>c/d</em>, it follows that

Multiply through everything on the right side by <em>b/d</em> to get

and so (<em>a</em> + <em>c</em>)/(<em>b</em> + <em>d</em>) < <em>c/d</em>.
For the other side, you can do something similar and divide through everything by <em>d</em> :

and <em>a/b</em> < <em>c/d</em> tells us that

Then

and so (<em>a</em> + <em>c</em>)/(<em>b</em> + <em>d</em>) > <em>a/b</em>.
Then together we get the desired inequality.