Complete and use a two-way probability table
A set of record sheets is classified by two binary properties
Event A means an item has a labelled corner; event B means an item is in the first row. The sample space is S, and the complements are A′ and B′
Event A is shown in the rows and event B is shown in the columns. Not A and Not B denote the complementary categories. The entries are counts of items
One item is selected uniformly at random from these recorded items
| B | Not B | Total | |
|---|---|---|---|
| A | 3 | 12 | 15 |
| Not A | ? | 5 | 10 |
| Total | 8 | 17 | 25 |