A-LevelMathematicsProbability and statistics9709/62 · May/June 2019 · Q24 Marks
The volume of ink in a certain type of ink cartridge has a normal distribution with mean 30 ml and standard deviation 1.5 ml. People in an office use a total of 8 cartridges of this ink per month.
- (a)Find the expected number of cartridges per month that contain less than 28.9 ml of this ink.[4]
Mark scheme
Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.
- [B1] Using ± standardising formula, no continuity correction, not σ² or √σ,
- [M1] Appropriate area Φ from standardisation formula P(z <....) in final probability solution, Must be a probability, e.g. 1 – 0.622 is M0
- [A1] Correct final probability rounding to 0.232. (Only requires M1 not B1 to be awarded)
- [B1 FT] FT using their 4 SF (or better) value, ans. rounded or truncated to integer, no approximation indicated. (Also accept 1 but not both)
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