$interactive-xapi-designer A bag contains n red marbles, 7 white marbles and 5 blue marbles.

A further 9 red marbles are added to the bag.
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The probability of picking a red marble is now 7 over 11.

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a

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Find the value of 

n

n

.

 

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Suggested Answer

Find the value of 

n

n

.

n

+

9

n

+

9

+

7

+

5

=

7

11

n+9+7+5

n+9

​

 = 

11

7

​

 

4

7

n

=

48

7

7

4

​

 n= 

7

48

​

 

7

4

n

=

48

4n=48

4

n

=

12

n=12

 

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[1]

b

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Without replacing the first marble, what is the probability of picking 2 red marbles?

 

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Without replacing the first marble, what is the probability of picking 2 red marbles?

21

33

×

20

32

33

21

​

 × 

32

20

​

 

35

88

88

35

​