Probability equation using arc4random()?

Question

In my game I know I can use conditions for this, but I wanted to see if there was a math equation that would make it easier on me. I'd like to use arc4random().

What I need is:

1. If my score is 1-20, probability: 1 out of 25
2. If my score is 21-40, probability: 1 out of 20
3. If my score is 41-60, probability: 1 out of 15
4. If my score is 60+, probability: 1 out of 10.

Is this possible? And if so, how would I achieve this?

Thanks!

Answer 1

This should do the trick (hope it's clear!):

int modNumber = 25;
float alteredScore = score - (floor((score-1)/60) - 1) * 60;
modNumber -= floor((alteredScore-1)/20) * 5;
int result = arc4random() % modNumber;  // Or arc4random_uniform(modNumber) if you want to completely remove modulo bias, but beaing in mind the non-randomness (!) of arc4random and the size of modNumber I highly doubt it would make any real difference
if (!result) {
    // Result is 0
    // Do stuff for the lucky person
} else {
    // Make them miserable
}