(b) How many different groups of 6 children can be chosen from a class of 18 children if the class contains one set of twins who must not be separated?

Interpreting as: different

Assuming "different" is a word
Input interpretation:
different (English word)

Definitions:
1 | adjective | unlike in nature or quality or form or degree
2 | adjective | distinctly separate from the first
3 | adjective | differing from all others; not ordinary
4 | adjective | marked by dissimilarity
5 | adjective | distinct or separate
(5 meanings)

Well if you have 20 bats you probably want 20 balls? you don’t have enough context to answer the question.