Let C = the Chinese books. Let E = the English language books.
C = 4/5 E
E - 300 = C - 180 Notice how you get this equation. You take 300 away from the English books and 180 from the Chinese and according to the question, you get an equal number of both. Put c = 4/5E into the second equation.
E - 300 = 4/5 E - 180 Add 300 to both sides. E = 4/5 E - 180 + 300 E = 4/5 E + 120 Now subtract 4/5 E from both sidies. E - 4/5 E = 120 1/5 E = 120 Now multiply by 5 E = 120 * 5 E = 600
So the number of books are 600 written in English 4/5 of 600 = 2400 /5 = 480 Chinese books.
