How many books did the author write?

Target question: How many books did the author write?

Statement 1: Of the books the author wrote, 3 were bestsellers.
Since we don't know how many books were NOT bestsellers, we can’t answer the target question with certainty.
Statement 1 is NOT SUFFICIENT

Statement 2: Of the books the author wrote, 80 percent were not best-sellers.
Here, we aren't told how many books the author wrote.
Consider these two possible scenarios:
Case a: The writer wrote a total of 10 books, 8 of which were not bestsellers, and 2 were bestsellers. In this case, the answer to the target question is the author wrote 10 books
Case b: The writer wrote a total of 20 books, 16 of which were not bestsellers, and 4 were bestsellers. In this case, the answer to the target question is the author wrote 20 books
Since we can’t answer the target question with certainty, statement 2 is NOT SUFFICIENT

Statements 1 and 2 combined
Statements 2 tells us that 80% of the books were not bestsellers, which means 20% of the books were bestsellers.
So, if we let N = the total number of books the author wrote, we know that: 20% of N = the number of bestsellers
In other words: The number of bestsellers = 0.2N

Statement 1 tells us that the author wrote 3 best sellers, which means our equation becomes: 3 = 0.2N
Divide both sides of the equation by 0.2 to get: N = 15
In other words, the author wrote 15 books.
Since we can answer the target question with certainty, the combined statements are SUFFICIENT

Answer: C