Probability: Dependent Events

First, let's see the 1st pick.

There are 5, 4, and 3 marbles in the jar.
So n(S) = 5 + 4 + 3.

5 + 4 = 9

9 + 3 = 12

So n(S) = 12.

This means,
for the 1st pick,
there are 12 marbles.

There are 5 blue marbles.
So n(A) = 5.

n(S) = 12

So P(A) = 5/12.

Probability

Next, let's see the 2nd pick.

It says
a marble is randomly picked from the jar
and [not replaced].

So the 1st pick (A)
does affect the 2nd pick (B).

So A and B are dependent events.

So, instead of finding P(B) by
P(B) = P(A) = 5/12,
find the changed probability P(B').

There were 5 blue marbles.
And 1 blue marble is picked and not replaced.

Then there are, 5 - 1, 4 blue marbles.
So n(B') = 4.

There were 12 blue marbles. (n(S) = 12)
And 1 marble is picked and not replaced.

Then there are, 12 - 1, 11 marbles.
So n(S') = 11.

n(B') = 4
n(S') = 11

So P(B') = 4/11.

P(A) = 5/12
P(B') = 4/11

A and B are dependent events.

So
P(A and B) = [5/12]⋅[4/11].

Cancel the numerator 4
and reduce the denominator 12 to, 12/4, 3.

3⋅11 = 33

So 5/33 is the answer.