Question

A bag has 7 red marbles, 8 blue marbles and 3 black marbles. What is the probability of picking a blue marble, replacing it, and then picking a red marble?
Answers:
2/27
7/108
1/9
14/81

Answer 1

What do you know about basic probability?

Answer 2

not a lot @mrbarry

Answer 3

You do know that the probability of an event can be expressed as a ratio / fraction.
The numerator is how many ways what you want could happen. The denominator is how many things could possible happen.

Flip a coin. Probability of heads is 1/2. 1 way to get heads over 2 possibilities.

Answer 4

18 @mangorox

Answer 5

Now you know the denominator.

Answer 6

but thats not the denominator for the answer

Answer 7

@mrbarry that's not the denominator for the answer!

Answer 8

True, but it is part of finding the probability you need. How many ways can you get a blue marble?

Answer 9

dont know

Answer 10

How many blue marbles does the problem say there are?

Answer 11

8

Answer 12

So the probability of getting the blue marble on the first draw is: 8/18 which simplifies to....

Answer 13

4/9

Answer 14

Right. Now, what about the next part. You have put that blue marble back. What is the probability of now drawing a red?

Answer 15

1/9

Answer 16

?

Answer 17

# of reds =
------------------------
# of marbles=

Answer 18

and that is?

Answer 19

That fraction is the probability of getting a red from the full bag of marbles. There are 7 red marbles, so the probability should be....

Answer 20

7/108 right?

Answer 21

7/18 you mean?

Answer 22

right I was looking in the answer key

Answer 23

You just did two experiments. If we want to know the probability of one and then the other, we have to multiply the two probabilities together (4/9)(7/18) =

Answer 24

14/81 @mrbarry

Answer 25

Looks good!