So, you know that the price of a single copy varies depending on the number of books that are being purchased.
More specifically, if you buy less than 20 books, you pay #"16 Rs"# per book, but that if you buy 20 or more books, you only have to pay #"13 Rs"# per book.
First, figure out how much money you have to pay when you buy 20 books.
#20color(red)(cancel(color(black)("books"))) * "13 Rs"/(1color(red)(cancel(color(black)("book")))) = "260 Rs"#
Now, you need to figure out the minimum number of books you that can buy at full price for more money.
If #n# rpresents the number of books, which you know must satisfy the condition #n<20#, then you have
#ncolor(red)(cancel(color(black)("books"))) * ("16"color(purple)(cancel(color(black)("Rs"))))/(1color(red)(cancel(color(black)("book")))) > 260color(purple)(cancel(color(black)("Rs")))#
#n * 16 > 260#
Divide both sides of the equation by #16# to get
#( n * color(red)(cancel(color(black)(16))))/color(red)(cancel(color(black)(16))) > 260/16#
#n > 65/4#
#n > 16.25#
The minimum number of books that you can buy at full price and still pay more than you would for a 20-book purchase at a discount is
#n = color(green)(17)#
