Your code has wrong syntax.

Sorry, missed this one: post under the Java translator comment here, so that he can be notified :)

Language and, possibly, code with a spoiler tag?

Erroneous letters are from n inclusive to z inclusive.

so number of erroneous letters and number of correct letters should be correct, right?

Not necessarily, one error on erroneous ones can compensate one corresponding error on correct ones...

The denominator should just be the length of the string. The numerator will be incremented by 1 each time you see a character that is greater than 'n'. Hopefully that helps enough for you to solve this one!