Coding pattern for random percentage branching?

So let's say we have a code block that we want to execute 70% of times and another one 30% of times.

if(Math.random() < 0.7)
    70percentmethod();
else
    30percentmethod();

Simple enough. But what if we want it to be easily expandable to say, 30%/60%/10% etc.? Here it would require adding and changing all the if statements on change which isn't exactly great to use, slow and mistake inducing.

So far I've found large switches to be decently useful for this use case, for example:

switch(rand(0, 10)){
    case 0:
    case 1:
    case 2:
    case 3:
    case 4:
    case 5:
    case 6:
    case 7:70percentmethod();break;
    case 8:
    case 9:
    case 10:30percentmethod();break;
}

Which can be very easily changed to:

switch(rand(0, 10)){
    case 0:10percentmethod();break;
    case 1:
    case 2:
    case 3:
    case 4:
    case 5:
    case 6:
    case 7:60percentmethod();break;
    case 8:
    case 9:
    case 10:30percentmethod();break;
}

But these have their drawbacks as well, being cumbersome and split onto a predetermined amount of divisions.

Something ideal would be based on a "frequency number" system I guess, like so:

(1,a),(1,b),(2,c) -> 25% a, 25% b, 50% c

then if you added another one:

(1,a),(1,b),(2,c),(6,d) -> 10% a, 10% b, 20% c, 60% d

So simply adding up the numbers, making the sum equal 100% and then split that.

I suppose it wouldn't be that much trouble to make a handler for it with a customized hashmap or something, but I'm wondering if there's some established way/pattern or lambda for it before I go all spaghetti on this.

Thanks!