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There are only \( r \) red counters and \( g \) green counters in a bag. A counter is taken at random from the bag. The probability that the counter is green is given. The counter is then put back in the bag.

Next, 2 more red counters and 3 more green counters are added to the bag. A counter is again taken at random from the bag. The probability that the counter is green is given.

Find the number of red counters and the number of green counters that were in the bag originally.

(5 marks)

Answer :

The original bag contained 12 red counters and 9 green counters.

Let’s break down the problem step by step:

1. Initial Scenario:

o Initially, there are r red counters and g green counters in the bag.

o The probability of drawing a green counter is given as 3/7.

2. After Putting the Counter Back:

o We put the counter back in the bag.

o Additionally, 2 more red counters and 3 more green counters are added to the bag.

o Now the total number of counters in the bag is (r + 2) + (g + 3).

3. New Probability:

o We want to find the probability of drawing a green counter from the updated bag.

o This probability is given as 6/13.

Let’s set up the equations based on the given information:

• Initial Probability:[tex][ P(\text{green}) = \frac{g}{r + g} = \frac{3}{7} ][/tex]

• Updated Probability:[tex][ P(\text{green}) = \frac{g + 3}{(r + 2) + (g + 3)} = \frac{6}{13} ][/tex]

Now let’s solve for the original number of red and green counters:

1. Initial Probability Equation:[tex][ \frac{g}{r + g} = \frac{3}{7} ] Solving for (g): [ 7g = 3r + 3g ] [ 4g - 3r = 0 \quad \text{(1)} ][/tex]

2. Updated Probability Equation:[tex][ \frac{g + 3}{(r + 2) + (g + 3)} = \frac{6}{13} ] Solving for (g): [ 13(g + 3) = 6(r + g + 5) ] [ 13g + 39 = 6r + 6g + 30 ] [ 7g - 6r = -9 \quad \text{(2)} ][/tex]

3. Solving the System of Equations: Multiply Equation (1) by 2 and subtract Equation (2): [ 8g - 6r = 0 ] [ 7g - 6r = -9 ] [ 8g - 7g = 9 ] [ g = 9 ]

4. Finding the Number of Red Counters: Substitute (g = 9) into Equation (1): [ 4g - 3r = 0 ] [ 4(9) - 3r = 0 ] [ 36 - 3r = 0 ] [ 3r = 36 ] [ r = 12 ]

Therefore, the original bag contained 12 red counters and 9 green counters.

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