A-LevelMathematicsProbability and statistics9709/51 · May/June 2021 · Q25 Marks
A company produces a particular type of metal rod. The lengths of these rods are normally distributed with mean 25.2 cm and standard deviation 0.4 cm. A random sample of 500 of these rods is chosen.
- (a)How many rods in this sample would you expect to have a length that is within 0.5 cm of the mean length?[5]
Mark scheme
Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.
- [M1] Use of ± Standardisation formula once; no continuity correction, σ², √σ
- [A1] For AWRT 0.8944 SOI
- [M1] Appropriate area 2Φ – 1 OE, from final process, must be probability
- [A1] Accept AWRT 0.789
- [B1FT] Correct or FT their 4SF (or better) probability, final answer must be positive integer, not 394.0 or 395.0, no approximation/rounding stated, only 1 answer
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