Standard +0.3 This is a straightforward application of the normal approximation to Poisson with scaling. Students must recognize that 300 days means λ = 5.1 × 30 = 153, apply the normal approximation Po(153) ≈ N(153, 153), use continuity correction (P(X < 140) = P(X ≤ 139.5)), and standardize. While it requires multiple steps, each is routine for S2 level with no novel insight needed—slightly easier than average due to clear structure.
3 The number of e-readers sold in a 10-day period in a shop is modelled by the distribution \(\operatorname { Po } ( 5.1 )\). Use an approximating distribution to find the probability that fewer than 140 e-readers are sold in a 300-day period.
3 The number of e-readers sold in a 10-day period in a shop is modelled by the distribution $\operatorname { Po } ( 5.1 )$. Use an approximating distribution to find the probability that fewer than 140 e-readers are sold in a 300-day period.\\
\hfill \mbox{\textit{CAIE S2 2018 Q3 [4]}}