From Monday to Tuesday, Nico reads a total of 20 + 12 = 32 pages. On Wednesday, he read a total of 51 - 32 = 19 pages.
This justifies what answer for what question? Q & A:
Nico borrows 3 books from the library on Monday. On Monday, he reads the first book with a total of 20 pages. On Tuesday, he reads the second book with a total of 12 pages. On Wednesday, he reads the third book. If he has read a total of 51 pages from Monday to Wednesday, how many pages did he read on Wednesday?
19