Answer:
The previous knowledge of when he was treating humans but kept losing money because of the pets he kept in his house.
Step-by-step explanation:
At the beginning of the book, the reader learns that the Doctor was first a human physician. He, however, kept a lot of pets which scared his human patients. They soon became so afraid that they stopped coming to his house to seek medical treatment. This made him begin losing so much money that his sister even complained about it.
This background history would help Sweet T infer that Dr. Dolittle was losing money because of the crocodile that scared pet owners away.