9514 1404 393
Answer:
 5/12
Step-by-step explanation:
Since Pedro had 1/3 of the book remaining at the end of the day, he had read a total of 2/3 of the book. If n represents the amount read at night, you have ...
 1/4 + n = 2/3
 3 +12n = 8 . . . . . multiply by 12
 12n = 5 . . . . . . . .subtract 3
 n = 5/12 . . . . . .  divide by 12
Pedro read 5/12 of the book at night.
_____
Since Pedro reads only a fraction of the book, it is not a "mixed number."
__
The solution is equivalent to finding the difference ...
 2/3 - 1/4
 = 8/12 - 3/12 = 5/12