The above is an estimation question. Estimation is when you judge the results obtained from a sample and make a rough calculation. The answer obtained might not be exactly right, its based on a judgement.
In this a sample was taken and out of 80 books, 6 of them were history books.
The assumption made then is that for every 80 books taken randomly , 6 of them will be history. Using this assumption ;
21 360 books in the library,
for every 80 books - 6 history books
then for 21 360 books - 6 / 80 * 21360
= 1602 history books
