[Maxandmov]

you can use pseudorandom for it.
e.g. let's say you want someone to have 15% chance to be successful in *put action name here*

Код:

new chance = random(100) //Random number (from 0 to 99)
if(chance <= 14) //15% chance of success
{
   ... //Do your actions here upon success
}
else
{
   ... //Do your actions here upon failure
}

Direct SAMP wiki to figure other functions you need out yourself.