If the total number of books in two schools is regarded as 1, then the original books in A and B schools account for 7/ 12 and 5/ 12 respectively.

Later, the books of the two schools accounted for 3/7 and 4/7 respectively, so it turned out that both schools had books:

650÷[(7/ 12)-(3/7)]

=650÷( 13/84)

=650×(84/ 13)

=4200

The original book of a school is 4200×(7/ 12)=2450.

B proofread the original book 4200-2450= 1750.

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