The first night he read 20% of his 500-page book so he read .20*500 = <<20*.01*500=100>>100 pages
The book is 500 pages and he read 100 so that leaves 500-100 = <<500-100=400>>400 pages
The second night he reads 20% of the 500 pages so he read .20*500 = <<20*.01*500=100>>100 pages
The book is now 400 pages and he read 100 so that leaves 400-100 = <<400-100=300>>300 pages
The third night he read 30% of the book so he read .30*500 = <<30*.01*500=150>>150 pages
The book is now 300 pages and he read 150 pages so he has 300-150 =<<300-150=150>>150 pages left to read
#### 150