Human: Xander read 20% of his 500-page book in one hour.  The next night he read another 20% of the book.  On the third night, he read 30% of his book.  How many pages does he have left to read? Assistant:
The first night he read 20% of his 500-page book so he read .20*500 = 100 pages
The book is 500 pages and he read 100 so that leaves 500-100 = 400 pages
The second night he reads 20% of the 500 pages so he read .20*500 = 100 pages
The book is now 400 pages and he read 100 so that leaves 400-100 = 300 pages
The third night he read 30% of the book so he read .30*500 = 150 pages
The book is now 300 pages and he read 150 pages so he has 300-150 =150 pages left to read