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4 books for each subject can be arranged as in shelf F so F (4, 4) therefore it can be written as 4! which means four factorial
4×3×2×1 = 24 ways
So the books can be arranged in 24 ways.
Given there are four books.

one chemistry ,one physics,one maths,one biology.

in first place any one of the four can be the second one of the remaining three  can be the third one of the remaining two can be kept.the remaining book is kept in the last.

4 x 3 x 2 x 1  = 24 = 4!