Complete and use a two-way probability table
A set of paper samples is classified by two binary properties
Event A means an item has a square mark; event B means an item is folded once. 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 | 12 | 5 | 17 |
| Not A | ? | 3 | 12 |
| Total | 21 | 8 | 29 |