Answer :
23245 + 21.50b = 46389 + 10.50b
21.50b - 10.50b = 46389 - 23245
11b = 23144
b = 23144/11
b = 2104 <== the cost will be the same at 2104 books
21.50b - 10.50b = 46389 - 23245
11b = 23144
b = 23144/11
b = 2104 <== the cost will be the same at 2104 books