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2.3 Probability

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Question 4

A box contains $n\,$ red counters and 6 blue counters, where $n\,$ is a positive integer. Two counters are taken from the box at random, without replacement.

The probability that both counters taken are blue is $18\displaystyle \frac{1}{8}$ .

Show that $n^2 + 11n - 210 = 0$ .

[3]

Hence find the value of $n$ .

[2]

Find the probability that the two counters taken are different colours.

[2]

Markscheme

2.3 Probability Questions

A Level
Maths
2.3 Probability

200 exam-style questions on OCR A Level Maths 2.3 Probability, covering 2.3.1 Mutually exclusive and independent events, 2.3.2 Diagrams to assist probability calculations, 2.3.3 Conditional probability (A-level only), 2.3.4 Conditional probability from first principles (A-level only), and 2.3.5 Modelling with probability. Each one has a worked solution and a mark scheme showing where the marks go.

2.3 Probability

Question bank

2.3 Probability

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Question 4

A box contains $n\,$ red counters and 6 blue counters, where $n\,$ is a positive integer. Two counters are taken from the box at random, without replacement.

The probability that both counters taken are blue is $18\displaystyle \frac{1}{8}$ .

Show that $n^2 + 11n - 210 = 0$ .

[3]

Hence find the value of $n$ .

[2]

Find the probability that the two counters taken are different colours.

[2]

Markscheme

2.3 Probability Questions

A Level
Maths
2.3 Probability