Contents of this Probability theory tutorial:

Random variable, Distribution, Distribution function, Discrete distribution, Continuous distribution, Discrete distribution function, Continuous distribution function.

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Probability theory tutorial

Comments

http://www.mathxplain.com/probability-theory/distribution-distribution-f...
X < x = x sq * PI / X sq * PI = x sq * PI / 50 sq * PI
for x = 10 it is smaller 0.04
for x = 20 it is 0.16, increases as radius increases - our chance of hitting increases
for x = 50 it is 1, does not this mean increases to max of hitting?
for x = 60 it is 60 sq / 50 sq, doesn't this mean we hit both inside (0 to 50) & possibly also outside too (50+ to 60)?... or is it definitely hit the target (as mentioned by you)?

Edit:

OK, got it, 1 is correct for x = 60, since we assumed "we definitely hit"

Mentioning "earlier assumption" as below, will help besides hinting with red color, as we are causual readers, may miss.

           "This is because of our earlier assumption "we definitely hit the target"  "