# Conditional probability help

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#1
Hi, basically ive been working through this questions and im not getting the right answer. Does anyone mind showing their workingg?? Thanks!! Q) In a childs game there should be 7 triangles 3 of which are blue and 4 of which are red, and 11 Squares, 5 of which are blue and 6 of which are red. However, two pieces are lost.

Assuming the pieces are lost at random, find the probability that they are
(i) the same shape
(ii) the same colour
(iii) the same shape and the same colour
(iv) the same shape given that they are the same colour.
0
4 years ago
#2
(Original post by pandasandpens)
Hi, basically ive been working through this questions and im not getting the right answer. Does anyone mind showing their workingg?? Thanks!! Q) In a childs game there should be 7 triangles 3 of which are blue and 4 of which are red, and 11 Squares, 5 of which are blue and 6 of which are red. However, two pieces are lost.

Assuming the pieces are lost at random, find the probability that they are
(i) the same shape
P(triangle) = 7/(7+11)
P(square) = 11/(7+11)

(i) = P(triangle 1) * P(triangle 2) + P(square 1)*P(square 2).
0
4 years ago
#3
i) Add up the number of shapes, and you get 19. First, find the probability that 2 are Triangles and add that to the probability that the 2 are Squares. You get:
7 x 6 + 11 x 10 which gives you 4
19 18 19 18 9
ii) Same process, but instead of using 7, 6 and 11, 10 you use 10, 9 and 8, 7
my calculator gives me a horrible number; 73 divided by 171

Basically, you work out the probability of getting what you want first time round, then work out getting what you want second time round and multiply, remembering to subtract 1 from the numerator and denominator for the second one since an item has already been taken out.
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