Math Exam on Monday... help please :)

Forgive me for bumping in here, without a hello-post... So consider this my hello ;) But I got math exam on monday and I'm getting quite desperate, since any of my friends can solve it either.

English isn't my native tongue and the exercise isn't in English but I did my best to translate it...

*The results of the entree exam for doctors follows a normal distribution. The max score is 600. After correcting the exams it seems 15% of the participants has less than 230 and 70% has less then 425.*

What is the average of the entree exam...

(Thanks to Moo for the correct manner of stating it in English)

The solution should be 359 according to the book, but I need to find out how you calculate it...

+Rep for everyone who helps!

Thx in advance :D

Edit: Just noticed I put this in the wrong place... :( should be in high school section... Blame me being frustrated because I can't find the solution :p