$$300\div20-300\div30=5$$. 
Alice began reading a 400-page book at 8 am and had read 50 pages by 9:30. If she continues to read at the same rate, when can she finish reading this book? 
Options:
(1) 6 pm 
(2) 7 pm 
(3) 8 pm 
(4) 9 pm 
(5) 10 pm 