Since there are 276 jelly beans of equal amount, split into 12 different colors, there will be 23 jelly beans per color. Mowing this, there will be 23 white jelly beans out of 276. This can be expressed by:
23/276 white jelly beans.
Reducing this fraction will give you
1/12 of a change to get a white jelly bean.
This, in a decimal format will be
0.0833333333 repeating or
0.1 % chance of getting a white jelly bean
