The number of faults found on each car in a batch is summarised in the table below, where a a\,a is a positive integer.
| Number of faults | 0 | 1 | 2 | 3 | 4 |
|---|---|---|---|---|---|
| Frequency | 10 | aaa | 18 | 9 | 3 |
The mean number of faults per car is exactly 1.5.
Find the value of aaa.
Find the standard deviation of the number of faults.
244 exam-style questions on AQA A Level Maths 2.2 L: Data presentation and interpretation, covering 2.2.1 Single-variable data diagrams, 2.2.2 Scatter diagrams and correlation, 2.2.3 Central tendency, variation and standard deviation, 2.2.4 Outliers and cleaning data, and 2.2 L: Data presentation and interpretation. Each one has a worked solution and a mark scheme showing where the marks go.