If you don't want to use algebra, writing "x" for the cost of the "five eighty page books", you a can argue this way:
To "undo" a calculation, you do the opposite- the opposite of "add 200" is "subtract 200" and the opposite of "multiply by five" is "divide by five".
A text book cost 400 Rs and that is "200 Rs more than five eighty page books". "200 more" means 200 has been added so the five eighty page books cost 400- 200= 200 Rs.
Since the five eighty page books costs 200Rs, a single eighty page book cost 200/5= 40 Rs.