statistics

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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?

• statistics -

http://davidmlane.com/hyperstat/z_table.html

I get below 50.3

• statistics -

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?

• statistics -

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