OCR MEI S1 2013 January — Question 4 7 marks

Exam BoardOCR MEI
ModuleS1 (Statistics 1)
Year2013
SessionJanuary
Marks7
PaperDownload PDF ↗
Mark schemeDownload PDF ↗
TopicCombinations & Selection
TypeProbability of specific committee composition
DifficultyModerate -0.8 This is a straightforward combinations question requiring basic nCr calculations and simple probability using favorable/total outcomes. Part (i) is direct application of C(11,3), while part (ii) requires calculating P(2 terriers) + P(3 terriers) using C(5,2)×C(6,1) and C(5,3), which is standard S1 material with no conceptual challenges.
Spec5.01a Permutations and combinations: evaluate probabilities5.01b Selection/arrangement: probability problems

4 At a dog show, three out of eleven dogs are to be selected for a national competition.
  1. Find the number of possible selections.
  2. Five of the eleven dogs are terriers. Assuming that the dogs are selected at random, find the probability that at least two of the three dogs selected for the national competition are terriers.

Part (i)
Answer: \(\binom{11}{3} = 165\)
AnswerMarks
MarksGuidance
M1Seen
A1cao
[2]
Part (ii)
Answer: \(\frac{\binom{5}{2} \times \binom{6}{1}}{\binom{11}{3}} + \frac{\binom{5}{3} \times \binom{6}{0}}{\binom{11}{3}} = \frac{60}{165} + \frac{10}{165} = \frac{70}{165} = \frac{14}{33} = 0.424\)
Alternative:
\(1 - P(1 \text{ or } 0) = 1 - 3 \times \frac{5}{11} \times \frac{10}{9} \times \frac{5}{9} \times \frac{6}{10} \times \frac{5}{11} \times \frac{4}{9}\)
\(= 1 - \frac{5}{11} - \frac{4}{33} = \frac{14}{33}\)
M1 for \(1 - P(1 \text{ or } 0)\), M1 for first product, M1 for second product, A1
AnswerMarks
MarksGuidance
M1For intention to add correct two fractional terms
M1For numerator of first term
M1For numerator of sec term. Do not penalise omission of \(\binom{6}{0}\)
M1For correct denominator
A1cao
[5]Or. For attempt at correct two terms. For prod of 3 correct fractions \(=4/33\). For whole expression ie \(3 \times \frac{5}{11} \times \frac{10}{9} \times \frac{6}{11} \left(\frac{-4}{11}\right) (= 3 \times 0.1212\ldots)\)
## Part (i)

**Answer:** $\binom{11}{3} = 165$

| **Marks** | **Guidance** |
|-----------|--------------|
| M1 | Seen |
| A1 | cao |
| **[2]** | |

## Part (ii)

**Answer:** $\frac{\binom{5}{2} \times \binom{6}{1}}{\binom{11}{3}} + \frac{\binom{5}{3} \times \binom{6}{0}}{\binom{11}{3}} = \frac{60}{165} + \frac{10}{165} = \frac{70}{165} = \frac{14}{33} = 0.424$

**Alternative:**
$1 - P(1 \text{ or } 0) = 1 - 3 \times \frac{5}{11} \times \frac{10}{9} \times \frac{5}{9} \times \frac{6}{10} \times \frac{5}{11} \times \frac{4}{9}$

$= 1 - \frac{5}{11} - \frac{4}{33} = \frac{14}{33}$

M1 for $1 - P(1 \text{ or } 0)$, M1 for first product, M1 for second product, A1

| **Marks** | **Guidance** |
|-----------|--------------|
| M1 | For intention to add correct two fractional terms |
| M1 | For numerator of first term |
| M1 | For numerator of sec term. Do not penalise omission of $\binom{6}{0}$ |
| M1 | For correct denominator |
| A1 | cao |
| **[5]** | Or. For attempt at correct two terms. For prod of 3 correct fractions $=4/33$. For whole expression ie $3 \times \frac{5}{11} \times \frac{10}{9} \times \frac{6}{11} \left(\frac{-4}{11}\right) (= 3 \times 0.1212\ldots)$ |

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4 At a dog show, three out of eleven dogs are to be selected for a national competition.\\
(i) Find the number of possible selections.\\
(ii) Five of the eleven dogs are terriers. Assuming that the dogs are selected at random, find the probability that at least two of the three dogs selected for the national competition are terriers.

\hfill \mbox{\textit{OCR MEI S1 2013 Q4 [7]}}