Creating cumulative distribution function of Random variable.

Electrons hit a circular plate with unit radius. Let X be the random variable representing

the distance of a particle strike from the centre of the plate. Assuming that a particle is

equally likely to strike anywhere on the plate,

(a) for 0 < r < 1 find P(X < r), and hence write down the full the cumulative distribution

function of X, FX;

(b) find P(r < X < s), where r < s;

(c) find the probability density function for X, fX.

(d) calculate the mean distance of a particle strike from the origin.

I have problem in crating CDF and finding mean distance.

please help me.