Asked by K.T

the average of final exam is 64, the standard deviation is 7. If the teacher set a rule that 1/4 of students who receive the poor grades should stay after class. lower than what mark should the students stay in school?

does anyone know how to solve this?
Answered by bobpursley
http://davidmlane.com/hyperstat/z_table.html

I get below 50.3

Answered by K.T
my friend came out with the following solution.

P(Z<-0.675)=0.25
-0.675= (X-64)/7

but i'm kinda confused.
where did the 0.675 come from?
Answered by bobpursley
Good question; ask your friend. It makes no sense. See the link I posted.