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.