3/5 and 2/3 can become 6/ 10 8/ 12.
It can be seen that the denominator of the latter is the denominator of the former +2, that is, the newly published 400 story books.
Therefore, 400 books account for 2/( 12-2)= 1/5, that is, the original books of the whole school divided by 1/5 equals 2000 books.
If the formula is simple, assuming there are X books in the whole school, just solve the following formula.
3/5*x+400=2/3*(x+400)
x=2000