How did you all find this question? I feel I wasn’t particularly well prepared to answer this one. It seems a bit out of syllabus. They use the fact that three consecutive numbers must include an even number and a multiple of three. It’s sort of intuitive when you think about but there’s no way someone could think of that during the test. It’s also not covered in the OUP book. Was it covered in whatever books you’re using or in your college?