What is the value of x in the figure below?

Again, nice problem. These are not super easy. You need to imagine the triangles in the figure and relate sides.

For instance: ADC, its hypotenuse is 15

For ABC, it is 20,

so 20/15 is the ratio between both triangles. Now let's choose the side with 'x'

x is one side of ADC, the corresponding side in ABC is 15 (the second longest sides). You need to rotate the triangles, etc ... That's the tricky part :)

So:

20/15 =15/x > x = 15*15/20 = 225/20 = 45/4

Which is C

Not sure whether it's too late to help you though

Also notice that it's kind of a product rule:

x * 20 = 15 * 15

I am sure the book explains a short way of getting to this equation, like sides opposite, blah blah, but what i told you is the reason. You asked for understanding it! Best luck!

The answer to your question is 1/9