He read 1/5 of the book on Friday
Here, we want to get the fraction of the book read on a particular day
From the question, we are told that 4/5 was left and 1/4 of this was what he read
For convenience sake, let us have a specific number of pages for the book
Let the number of pages the book have be 200
He has 4/5 unread
The number of pages left to read will be;
Now, he proceeded to read 1/4 of this
The number read will thus be;
Now, we want to calculate the fraction of the book that corresponds to this
Mathematically, that would be;