On Monday, there were 50 visitors. On Tuesday, there were twice as many, so 2*50 = 100 visitors
The total number of visitors after Tuesday will be 100+50 = <<100+50=150>>150 visitors.
For the remaining 5 days, an average of 20 visitors attended, giving a total of 5*20 = <<5*20=100>>100 visitors.
The total number of visitors who visited the library for the week was 100+150 = <<100+150=250>>250 visitors.
#### 250