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Balls in a bag, probability involving selections etc.

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New member
Nov 7, 2012
HELP PLEASE! My friend is suffering huge depression and stress and she really needs to hand in this assignment or she will fail... please if you can answer any of these questions!

A bag contains x pink balls and y yellow balls. Three balls are randomly selected from the bag.

a) Write down an expression for the probability that the three balls chosen were pink.

If there had been one more pink ball, then the probability that the three balls chosen were pink would have been double the answer found in a).
Show that y = (x^(2 )- x-2)/(5-x) .

Sketch a graph of x against y, and determine all the possible numbers of pink and yellow balls.

In a mixed class, there are 10 girls.
If 2 pupils are selected at random, the probability that they are both girls is 0.15
How many boys are in the class?

Three boxes are labelled A, B and C.
A contains 3 cubes, 4 pyramids and 5 spheres.
B contains 2 cubes, 1 pyramid and 7 spheres.
C contains 4 cubes, 5 pyramids and 6 spheres.
A shape is taken at random from each of the three boxes. Find the probability of
a) 3 cubes
b) a pyramid from A, a sphere from B and a pyramid from C
c) two pyramids and a sphere
d) no spheres
e) at least one sphere and no pyramids
f) no cubes or spheres
g) 3 cubes or 3 spheres
h) Either a cube or a sphere from A, a pyramid from B and either a cube or a pyramid from C

a) If 3 people are asked on which day of the week they were born, find the probability that
i) 2 or 3 are the same
ii) only 2 people are the same

b) If 4 people are asked the same question, find the probability that 2 or more were born on the same day.

Comment on the following arguments, identifying precisely any fallacies in the arguments.

On every day of the year, it either rains or it doesn’t. Therefore the chance that it will rain tomorrow is 1/2.
When answering a multiple- choice test in which there are four possible answers given to each question, the chance that Gemma answers a question correctly is 1/4.
A bag contains a number of red, blue and white beads. If you choose one bead at random from the bag, the probability that it is blue is 1/3 .
Make up two “myths” of your own.
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"Read Euler, read Euler." - Laplace
MHB Math Helper
Feb 29, 2012
Re: Urgent help please

Hello and welcome to MHB! :) I'm very sorry for your friend. :( However, it is not our policy to solve lists of exercises for others. We believe everyone benefits most from a discussion, and we're all willing to guide you to a greater understanding and the answers. To do that, it would be best if you created separate threads for each problem and posted your attempts at solutions. :D We will be happy to put you in the right track then.

Cheers, and hope for the best.


Staff member
Jan 26, 2012
Re: Urgent help please

Along the same lines as Fantini said, pick one problem and show an attempt at it. Also, we won't give help with problems that are graded if the person's teacher/professor isn't ok with you getting help.

I'm sorry your friend is having issues with depression and we will be glad to help him or her understand math better, but we need you to do it on our terms which will actually benefit your friend the most in the future. :)

Long story short - pick one problem, show some work and convince us that we aren't helping you cheat.

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