It’s 17 to 9 (Official clock system).

20:43

It’s 17 to 9.

Correct answer

20:42

It’s 18 to 9.

Error: 1 minute earlier

19:49

It’s 11 to 8.

Error: read minutes starting from hour markings and adding minutes

21:40

It’s 20 to 10.

Error: confuse hour and minute hands