Question

Suppose a bag of marbles has 4 green, 2 red, 5 yellow, 1 brown, and 7 blue marbles. What is the probability of picking a red marble?

Answer 1

Assuming that each marble can be picked with equal probability, we notice that there is a total of

$4+2+5+1+7 = 19$

marbles, of which 2 are red.

So, the probability of picking a red marble is

$\dfrac{2}{19}$

In fact, as in any other case of (finite) equidistribution, we used the formula

$P(\text{event}) = \dfrac{\text{number of favourable cases}}{\text{number of all possible cases}}$