Given :

The school librarian could buy 80 books casting Rs 90 each from the local bookseller and also given that the publisher offered her a 20% discount.

To Find:

The number of copies of Mathematics books which she could buy from the publisher for the sanctioned money.

Solution:

The cost of a Mathematics books= Rs 90

The cost of 80 Mathematics books= 80×90 = Rs 7200

The publisher offered her a 20% discount.

Then the discount on each Mathematics book= 90×(20/100)= Rs 18

Therefore after the discount cost of each book = 90-18 =Rs 72

Therefore the number of copies of Mathematics books which she could buy from the publisher for the sanctioned money = Total amount / cost of each book

=7200/72

=100

Thus the number of copies of Mathematics books is 100.