Math.Random() Setting Limits

So I want a random number from 3-10 or 4-6, but that the lower the numbe the more chances it has to be selected, meanwhile the higher is the rarest, how do I do that?

private int bonusPoints;
private double randomBonusPoints = Math.Random() * 100;

bonusPoints = (int)randomBonusPoints;

What else could I add to limit the numbers it gives me?