Physics is Phun said:
ok thanks. its a candian lottery Jcsd.
I've got another question not really the same but has to do with choosing so i'll put it here. It was a question I had on a quiz today. Hopefully I got it right
There is a committee to consist of 5 people if there are 15 students and 18 teachers to choose from and the committee needs atleast one student and one teacher how many different committees can be formed.
I believe i did 15c1 * 18c1 * 31c3
is this right?
You're overcounting. If the students are labelled 1, 2, 3,... and the teachers A,B,C,D,.. then the committee 1,A,2,3,4, corresponding to selecting 1 and A as your guaranteed teacher & student and 2, 3, 4 as the 3 chosen from the 31 people left, is counted again as 2,A,1,3,4, where 2 and A are your guaranteed student and teacher and 1,3,4 are from the 31 remaining. (note: the order I've used for the committee is supposed to reflect the reasoning behind your answer)
You could try to correct this overcounting or you could take a different approach. You have 4 acceptable possibilities to make #of students+#of teachers=5, namely 1+4, 2+3, 3+2, and 4+1. Find the number of committees in each case and add.