In part 1, we know that 1 extra book is equal to either $200, $300, or $600 because she is either 1 book, 2 books, or 3 books over the threshold.

In part 2, we know that her total revenue from selling 10 encyclopedias is GREATER THAN 4000. A fixed part of her revenue is $1000, so we subtract that to get the variable part:

Her Variable revenue for 10 encyclopedias is GREATER THAN 3000.

Therefore, her variable revenue for 1 encyclopedia is GREATER THAN 300.

Because from part 1 we know that the variable revenue from each book could be one of only three options, and because only one of those options is GREATER THAN $300 per book, we know that she gets paid a base of $1000 and then $600 per book for each book over the threshold.

However, here's the trick... We know that if she had sold THREE fewer in march, then she would have made $600 less.. which means that she sold only ONE encyclopedia over the threshold. That means she only gets paid for ONE encyclopedia, plus her base pay. Even though we don't know what the threshold is, we know that $1000 + $600 = $1600, her salary in March.

C does it.