Hi all , I need some idea about how to write a probability expression, what expression function is related ?

for example:

$TX is between 1 - 20;
rand($TX) ;
90% of rand($TX) is greater than 15 ;
10% of rand($TX) is smaller than 15 ;



anyone have any idea ?

cheers !

untested:
if(rand($PT/$NPT) < 0.9,
fit01(rand($PT/$NPT+$NPT), 15, 20),
fit01(rand($PT/$NPT+$NPT), 1, 15))

thanks edward! i will try it out ~!