There would be 5184 ways in which each subject books can be selected.
Step-by-step explanation:
Number of geography books = 5
Number of chemistry books = 4
Number of Science books = 7
Different ways of selecting geography books = 5!
= 5 X 4 X 3 X 2 X 1
= 120
Different ways of selecting chemistry books = 4!
= 4 X 3 X 2 X 1
= 24
Different ways of selecting Science books = 7!
= 7 X 6 X 5 X 4 X 3 X 2 X 1
= 5040
Total different possible ways they can select them = 120 + 24 + 5040
= 5184