10, 11, and 12 are 3 options out of the 12 sides. That is 3 out of 12, which is 3/12. If you simplify that, it is 1/4. In decimal form it is 0.25, and in percent form it is 25%. Hope this helps!
There is a 1 in four chance of rolling those 3 numbers. You know this because there is a 3/12 chance of of rolling a 10,11, or 12. Simplify the fraction and you get 1/4.
