Home / IBDP Maths analysis and approaches Topic: SL 4.6 Use of Venn diagrams, tree diagrams, counting principles HL Paper 1

IBDP Maths analysis and approaches Topic: SL 4.6 Use of Venn diagrams, tree diagrams, counting principles HL Paper 1

Question
A box contains 4 red balls and 2 green balls. Two balls are drawn at random.

(a) Let \( A \) be the event that the first ball drawn is red, and \( B \) be the event that the second ball drawn is green. Find \( P(A \cap B) \). [3]
(b) Let \( C \) be the event that exactly one of the two balls drawn is red. Find \( P(C) \). [3]
▶️ Answer/Explanation
Solution
(a) Calculating \( P(A \cap B) \):
\[ P(A) = \frac{4}{6} = \frac{2}{3} \]
After drawing one red ball, there are 5 balls left (3 red and 2 green):
\[ P(B|A) = \frac{2}{5} \]
\[ P(A \cap B) = P(A) \times P(B|A) = \frac{2}{3} \times \frac{2}{5} = \frac{4}{15} \]
✅ Answer: \(\frac{4}{15}\)

(b) Calculating \( P(C) \):
There are two ways to get exactly one red ball:
1. First ball red and second ball green: \( \frac{4}{6} \times \frac{2}{5} = \frac{4}{15} \)
2. First ball green and second ball red: \( \frac{2}{6} \times \frac{4}{5} = \frac{4}{15} \)
Total probability: \( \frac{4}{15} + \frac{4}{15} = \frac{8}{15} \)
✅ Answer: \(\frac{8}{15}\)
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