Question

You randomly draw a marble that contains 3 blue marbles, 4 green marbles, and 5 red marbles.

Answer 1

In cases like this, i.e. when you could pick every marble with the same probability, you compute the probability of picking a specific marble colour by dividing the number of favourable outcomes by the total number of outcomes.

The number of favourable outcomes is 3, because there are 3 blue marbles.

The total number of outcomes is 12, because there are 12 marbles in total.