Number Questions

All question words in Lojban are requests to fill in an unknown value: ma asks for an unknown sumti, and mo for an unknown selbri. In Lojban xo is the question word for numbers. So, remembering the sentence re lo mi ci mensi cu nelci la rikis.martin., how would I answer the following question?

xo le mensi cu nelci la rikis.martin.

The answer, of course, is re. (But not all questions that can be answered with a number have to take xo, as we'll see in the next lesson).

Tip: xo is also used in mathematics, as in

li ci su'i vo du li xo
3 + 4 = ? 

A few more examples:

Note: It is not actually necessary to include the lo after xo. In fact, it isn't necessary after any number — for example ci lo gerku could be simply ci gerku, if you prefer. However, some Lojbanists prefer to keep the lo for the sake of clarity.

A final question

Lojban has no difference between singular and plural: the dog and the dogs can both be le gerku. But suppose you wanted to make a distinction between the two; how would you do it?