When looking at probabilities, two ideas are always true.
1) Any probability is more than 0.
2) The sum of all the probabilites is 1.
Idea #2 works here. (For example, think of how a die has six things and the probability of each is 1/6. So 1/6 + 1/6 + 1/6 + 1/6 + 1/6 + 1/6 = 1.) Let G = the probability of grape, C = probability of cherry and O = the probability of orange. From Idea #2, G + C + O = 1. Since we know G and C, then
3/10 + 1/5 + O = 1.
3/10 + 2/10 + O = 1
5/10 + O = 1
O = 5/10
Thus, the probability of an orange jelly bean is 5/10 = 1/2.
