Probability that the selected student will be in an odd-numbered grade

[vvvidenov]

Homework Statement

A school contains students in grades 1,2,3,4,5 and 6. Grades 2,3,4,5 and 6 all contain the same number of students, but there are twice this number in grade 1.

a)If a student is selected at random from a list of all the students in the school, what is the probability that he will be in grade 3?
b)What is the probability that the selected student will be in an odd-numbered grade?

Homework Equations

The Attempt at a Solution

Let n=the size of grade(s) 2,(3,4,5,6) then 2n=the size of grade 1.
So the size of the sample space I don't know how to find.
I really need help on this. I am not sure what to do from here at all. Thanks.

 

[phreak]

This is really just testing how you work with variables. Since you have n = the size of grades 2,3,4,5,6, and 2n = size of grade 1, the school's population must be 2n + 5*n, where the 2n represents the size of grade 1, and the 5*n represents the size of grades greater than 1 (since there are 5 of them). Therefore, the population is 7n. Given this fact, how would you be able to find the probabilities of a) and b)?

 

[vvvidenov]

ok, I see the response above.

 

[vvvidenov]

so the event (student chosen is in grade 3) has size=7
so the probability is =1/7