statistics
posted by K.T on .
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?

http://davidmlane.com/hyperstat/z_table.html
I get below 50.3 
my friend came out with the following solution.
P(Z<0.675)=0.25
0.675= (X64)/7
but i'm kinda confused.
where did the 0.675 come from? 
Good question; ask your friend. It makes no sense. See the link I posted.