# Thread: red and black cubes

1. ## red and black cubes

I've got a maths exam next week and i need to know how to work this question out for my revision so please could u show me:

There are some cubes in a bag. The cubes are eithert red (R) or black (B).
The teacher says:
If you take a cube at random out of the bag, the probability that it will be red is 1/5.
(a) what is the probabilty that the cube will be black?

(b) A pupil takes one cube out of the bag. It is red. What is the smallest number of black cubes there could be in the bag?

(c) Then the pupil takes another cube out of the bag. It is also red. From this new information, what is the smallest number of black cubes there could be in the bag?

(d) A different bag has blue (B), green (G) and Yellow (Y) cubes in it. There is at least one of each of the three colours.
The teacher says:
If you take a cube at random out of the bag, the probability that it will be green is 3/5.
There are 20 cubes in the bag. What is the greatest number of yellow cubes there could be in the bag?

Please can you show your working for all of the questions so i understand it.

2. Originally Posted by molly
I've got a maths exam next week and i need to know how to work this question out for my revision so please could u show me:

There are some cubes in a bag. The cubes are eithert red (R) or black (B).
The teacher says:
If you take a cube at random out of the bag, the probability that it will be red is 1/5.
(a) what is the probabilty that the cube will be black?

$\displaystyle \color{red}{P(\text{black}) = 1 - 1/5 = 4/5}$

(b) A pupil takes one cube out of the bag. It is red. What is the smallest number of black cubes there could be in the bag?

$\displaystyle \color{red}{4. \text{ See above.}}$

(c) Then the pupil takes another cube out of the bag. It is also red. From this new information, what is the smallest number of black cubes there could be in the bag?

$\displaystyle \color{red}{\text{Smallest now is... Red 2/10 so Black is 8/10. The answer is 8.}}$

(d) A different bag has blue (B), green (G) and Yellow (Y) cubes in it. There is at least one of each of the three colours.
The teacher says:
If you take a cube at random out of the bag, the probability that it will be green is 3/5.
There are 20 cubes in the bag. What is the greatest number of yellow cubes there could be in the bag?

$\displaystyle \color{red}{(3/5)\times(20).}$

Please can you show your working for all of the questions so i understand it.
aNon1

3. Originally Posted by Anonymous1
aNon1
wats aNon1?

4. Originally Posted by molly
wats aNon1?
My name, ofcourse.

5. On question (c) where did u get the /10 bit from?

6. Originally Posted by molly
On question (c) where did u get the /10 bit from?
We know the probability is 1/5...

We know there are at least 2...

2/10=1/5