There is many ways to use it and i recomend you to see this documentation about String Format.
http://developer.android.com/intl/pt-br/reference/java/util/Formatter.html
But, if you need only one variable, you'll need to use %[type] where [type] could be any Flag (see Flag types inside site above). (i.e. "My name is %s" or to set my name UPPERCASE, use this "My name is %S")
<string name="welcome_messages">Hello, %1$S! You have %2$d new message(s) and your quote is %3$.2f%%.</string>
Hello, ANDROID! You have 1 new message(s) and your quote is 80,50%.