Joint probability density help

[teramerapyar]

Really stuck on this question. Help would be greatly appreciated.

Suppose that X is uniformly distributed on [0, 1] and that Y is uniformly distributed on
[0,X]. What is the joint density for X and Y ?

 

[royhaas]

\(\displaystyle f(x,y) = f(y|x)f(x), 0<y<x<1\).