Answer: the combined time he spent studying is 281/36 hours
Explanation:
The first step is to convert all mixed numbers to improper fraction.
On Monday Billy spent 4 1/4 hour study. Converting 4 1/4 hours to improper fraction, it becomes 17/4 hours.
On Tuesday he spent another 3 5/9 hour study. Converting 3 5/9 hours to improper fraction, it becomes 32/9 hours.
The combined time he spent studying answer as a mixed number would be
17/4 + 32/9 = (153 + 128)/36
= 281/36 hours