You can use " Set Variable: Set to Random Number " and set a variable to 1 - 100 or something higher if you want larger selection range.
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Then, place an " If Variable: Value Is Within Range " and put the variable you made with the random number along with two numbers.
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If you want to have a 50% chance of something, put the variable specified above, the number 1, and the number 50.
You can place multiple of the If Variables on a single code line, just make sure none of the Within Ranges intersect with others.
For example, have 1-50 in one " If Variable: Is Within Range " and have 51-100 in another.
I hope this helps!