A bag contains only red, green, brown and yellow marbles.
The probabilities of selecting each colour are shown below.
The probability of choosing a red marble is the same as choosing a yellow marble.
Colour Red Green Brown Yellow
Probability 0.1 0.72
Find the probability of selecting a brown or yellow marble.
You have stated only 2 probabilities , I will assume they are for the red and green, that is:
red --- .1
green -- .72
brown --- x
yellow --- .1
they must add up to 1, ....
.1 + .72 + x + .1 = 1
x = .08
so prob(brown or yellow) = .08 + .1 = .18