This science and technology book is named subject 1, subject 2, subject 3 and subject 4.
In your calculation method, there may be such a situation:
1.( 1) Five history books, choose 1, choose 1.
(2) For four books on science and technology, choose 1 and 1.
(3) Of the remaining 7 books, choose 2 books, and choose History 2 and Section 2.
(1) Of the five history books,1was chosen, and History 2 was chosen.
2) For four books on science and technology, choose 1 and choose section 2.
(3) Choose two of the remaining seven books, history 1 and discipline 1.
According to your calculation method, these are two options and calculation is two options.
However, in both cases, the four books, History 1, Discipline 1, History 2 and Discipline 2, are actually selected, that is to say, there is actually a selection method of 1, so I will repeat it here. Of course, the answer is wrong.
I think it should be calculated as follows:
(1) Choose 1 for history and 3 for science and technology, which means 1, 5*C, 3,4 = 5 * (4 * 3 * 2/3 * 2 *1) = 20.
(2) Choose two books on history and two books on science and technology, which are C, 2, 5*C, 2, 4 = (5 * 4/2 *1) * (4 * 3/2 *1) =10 * 6 = 60.
(3) Choose 3 books for history and 1 book for science and technology, that is, 3,5 * c on C, 1, 4=(5*4*3/3*2* 1)*4=40.
These three situations are not repeated, so the total * * * is 20+60+40= 120.