There are three choices of jellybeans - grape, cherry and orange. if the probability of getting a grape is 3/10 and the probability of getting cherry is 1/5, what is the probability of getting orange

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.