Finding probability, Uniform distribution
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Solution 1
Draw the square $[0,1]\times[0,1]$ and the region $X>Y$ to see the answer.
Solution 2
A more general approach is based on symmetry  since $X,Y$ have the same distribution and $\mathbb{P}[X=Y] = 0$ (here, because both are continuous random variables), any outcome $(x,y)$ where $x>y$ is just as likely as the outcome $(y,x)$, so $X>Y$ exactly half the time.
Note that this is independent of distribution  as long as $\mathbb{P}[X=Y] = 0$.
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Comments

Tarek Abed 10 months
What is the probability P(X>Y) given that X,Y are Uniformly distributed between [0,1]?

Tarek Abed over 10 yearsif that so then it will be conditional probability, which is not. if I would solve it the way you suggested above, I have to choose X Or Y before calculating. if I choose X first the probability would be X. if i choose Y first the probability would be 1Y.

Tarek Abed over 10 yearsThat might work, I will try, thanks a lot.

gt6989b over 10 years@TarekAbed One way is to integrate over all possible choices of $X$. What Emanuele did is, she showed you how to visually see it  the square is a set of all possible choices for $(X,Y)$ with each region being as likely an occurrence as its area (because of the uniform distribution for both $X$ and $Y$). So of all possible choices, you need the proportion where $X>Y$, as she indicated in the answer  it is exactly 1/2...