This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
The reason the value is 40 instead of 30 is because it uses the Derived class version of the addValue method in both calls so it is 20+20 which is 40 instead of the 20+10. This deals with the method being overridden in the subclass.