### 12. Number questions

The following cmavo is discussed in this section:

```      xo      PA      number question
```
The cmavo “xo”, a member of selma'o PA, is used to ask questions whose answers are numbers. Like most Lojban question words, it fills the blank where the answer should go. (See Chapter 19 for more on Lojban questions.)
```12.1)  li re su'i re du li xo
The-number 2 plus 2 equals the-number what?
What is 2 + 2?

12.2)  le xomoi prenu cu darxi do
The what-number-th person hit you?
Which person [as in a police lineup] hit you?
```

“xo” can also be combined with other digits to ask questions whose answers are already partly specified. This ability could be very useful in writing tests of elementary arithmetical knowledge:

```12.3)  li remu pi'i xa du li paxono
The-number 25 times 6 equals the-number 1?0
```
to which the correct reply would be “mu”, or 5. The ability to utter bare numbers as grammatical Lojban sentences is primarily intended for giving answers to “xo” questions. (Another use, obviously, is for counting off physical objects one by one.)