The answer 0.12 represents the portion of the books in the library that are how-to books on electronics.
This is calculated as follows:
0.3 (the portion of all books that are how-to books) multiplied by 0.4 (the portion of the how-to books that deal specifically with electronics) gives you 0.12. Thus, 0.12 of all books in the library are how-to books on electronics.