Poisson approximation with parameter finding

Questions that require finding an unknown parameter (typically n) given a probability condition, using the Poisson approximation to binomial.

1 questions · Standard +0.3

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CAIE S2 2021 June Q5
9 marks Standard +0.3
5 Most plants of a certain type have three leaves. However, it is known that, on average, 1 in 10000 of these plants have four leaves, and plants with four leaves are called 'lucky'. The number of lucky plants in a random sample of 25000 plants is denoted by \(X\).
  1. State, with a justification, an approximating distribution for \(X\), giving the values of any parameters.
    Use your approximating distribution to answer parts (b) and (c).
  2. Find \(\mathrm { P } ( X \leqslant 3 )\).
  3. Given that \(\mathrm { P } ( X = k ) = 2 \mathrm { P } ( X = k + 1 )\), find \(k\).
    The number of lucky plants in a random sample of \(n\) plants, where \(n\) is large, is denoted by \(Y\).
  4. Given that \(\mathrm { P } ( Y \geqslant 1 ) = 0.963\), correct to 3 significant figures, use a suitable approximating distribution to find the value of \(n\).