how do u find probability?

By looking under random rocks?

Seriously, probabality is the ratio of the number of possible successes divided by the number of total possibilities.

At a dance, there are 13 boys, and 3 girls.

The probability of a male female couple is
3/16 * 13/15 Notice in each instance of fraction, it is the number of successes available divided by the number total. In the second fraction, the number total has decreased one, because a girl is removed.

There are are many ways to "do" probabllity, depending on what is given.

See:

http://en.wikipedia.org/wiki/Probability