3 were called on Friday.
Then the each of the 3 called 3 others which all equals to 9, since 3*3 is 9.
Now, each of the 9 called 3 others on Sunday which altogether equals 27, since 9*3 is 27. Therefore, the answer is 27.
Every time the word each is used in a math problem, that would mean that you need to multiply something. And that's what happened here
Let 
be the random variable for the number of marks a given student receives on the exam.
10% of students obtain more than 75 marks, so
where 
follows a standard normal distribution. The critical value for an upper-tail probability of 10% is
where 
denotes the CDF of , and 
denotes the inverse CDF. We have
Similarly, because 20% of students obtain less than 40 marks, we have
so that
Then 
are such that
and we find
352 divided by 8=44
Each school will get 44 books.
