Just reply to a F.5C student on a basic core mathematics question (on P.5.38):
There are 8 outstanding students from junior forms and 9 outstanding students from senior forms in a school this year. 5 out of these 17 students are now selected for an overseas exchange programme. Find the number of combinations of selecting at least 1 student from junior forms and 1 from senior forms.
Here is the ‘so-called’ solution from a student:
as the student claimed, select 1 from junior, ways; select 1 from senior, ways; then select the remaining 3 students from the remaining 15 students, ways, hence, the total number of combination should be , right?
Sorry, it is incorrect. (more…)