The times taken by Obi to walk to work each morning are normally distributed with mean 14.8 minutes and standard deviation 1.5 minutes.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2025). The mark or mark type is shown in brackets.

1. [M1] Use of ± standardisation formula with 15.6, 14.8 and 1.5 substituted, condone σ², condone √σ, condone continuity correction of ±0.05.
2. [M1] Calculating the appropriate probability area (leading to their final answer, expect < 0.5).
3. [A1] AWRT 0.297. If one or both M marks not awarded, SCB1 for AWRT 0.297 WWW.
4. [B1] -1.282 or 1.282 seen, CAO – critical value.
5. [M1] Using ± standardisation formula with 14.8, 1.5 and t equating to a z-value (not 0.8159, 0.1841, 0.9, 0.1, 0.5398, 0.4602 ...) not σ², not √σ, no continuity correction.
6. [A1] AWRT 12.9.
7. [M1] Substituting 14.8 and 1.5 and either 13.8 or 15.8 appropriately into one ±standardising formula, not o² or √o, no continuity correction. SCM1 – Condone substituting 14.8 and 1.5 and either 14.3 or 15.3 appropriately into one ±standardising formula, not σ² or √σ, no continuity correction.
8. [M1] Calculating the appropriate probability area (leading to their final answer, expect < 0.5) from their Z-values.
9. [A1] AWRT 0.495. If one or both M marks not awarded, SCB1 for AWRT 0.495 SOI.
10. [B1FT] Strict FT their at least 4 figure probability × 225 (Check with calculator). One integer answer, expect 111 or 112. No indication of ‘approximation', e.g. ≈, =, about, 3SF.