In the toy store, there are large and small plush kangaroos. Altogether, there are 100 of them. Some of the large kangaroos are female kangaroos. Each female kangaroo has three small kangaroos in her pouch, and the other kangaroos have empty pouches. Determine how many large kangaroos are in the store, given that there are 77 kangaroos with empty pouches.

(Bednářová)
If 77 kangaroos have an empty pouch, then $100-77=23$ kangaroos have a full pouch. Therefore, there are 23 kangaroos in the store.

Each kangaroo has 3 baby kangaroos in their pouch. 23 kangaroos have a total of $3 \cdot 23=69$ baby kangaroos in their pouches.

The number of adult kangaroos in this store is then $100-69=31$.

#