The answer 0.12 represents the portion of the books in the library that are how-to books on carpentry.
This is calculated by taking 0.3 (the proportion of books that are how-to books) and multiplying it by 0.4 (the proportion of how-to books that deal with electronics). Hence, 0.12 indicates the fraction of the total library collection that consists specifically of how-to books on carpentry.