Uniform Distribution/Probability Density Function

[ambiguous325]

I'm experiencing a mental block with this concept. If anyone could help me out, I'd be most appreciative.

I understand the formula for the function is: f(x)= 1 / d-c.

My problem comes in trying to apply this to a problem:

Problem: Suppose x is a random variable best described by a uniform probability distribution with c=20 and d=45.
a) find P(20<=x<=30)

For this problem, I am totally lost as far as what to plug in where. My best guess would be:
(b-a) / (d-c) = (30-20) / (45-20)

b)P(x>=30)

Would this be:
(d-c) (1/(d-c)) = (45-20) (1/(45-20))

It just doesn't seem right to me.....

Thanks for any help!

[JohnM]

(a) is correct

(b) is (45-30)/(45-20) = 15/25 = 3/5 = 0.6

[ambiguous325]

Thanks for the response.

Still a little confused, so I guess I'll bring it up in class tonight. :)

[JohnM]

It's exactly like determining the area of a rectangle - the questions are merely asking you to draw a vertical partition in the "whole" rectangle, and then determine the area of the remaining, "smaller" rectangle.