Help on a probabilty problem, HELP

From: bin (binye1973_at_yahoo.com)
Date: 11/18/04 

Date: Wed, 17 Nov 2004 18:12:52 -0600

Consider a particular Ph.D. student; we know that after she begins her Ph.D.
program, the number of years it takes her to complete her studies is a
random
variable with distribution exp(1/4) (independent of when she started).
Suppose
you know that the student completed her Ph.D. today. We wish to estimate how
long ago she started. Assume that the a priori distribution of X is uniform
on
[3, 6].

a. Let X represent the number of years ago the student started. Let Y
represent the observation of when she completes, relative to today (so the
given observation is Y = 0).
Find fY | X(y | x) for x >= 0.

can not understand the meaning very precisely ,Can not understand the
relation of exp(1/4) and uniform [3,6] , which is X ? which is Y ?thanks

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