Teacher shoxx... students roxx... ;)

Teacher: 1 Book + 1 Book ?
Student: 2 Books
.
Teacher: 2 Books + 2 Books ?
Student: 4 Books
.
Teacher: (now i'll ask U a tough 1)!
61,789,365 Books + 23,678,989 Books ?? :x
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Student: ..LIBRARY.. :D :P