Answer:
The answer is 13 days.
Step-by-step explanation:
Let's call the days he can work in the library x.
The days he can work in construction work is 20-x. Then x days per 5$ plus (20-x) days per 9$ should be bigger than 125$.
[tex]5x+9(20-x)>125[/tex]
[tex]5x+180-9x>125[/tex]
[tex]55>4x[/tex] therefore [tex]13,75>x[/tex]
Since x must be integer, the biggest we choose is 13.
To crosscheck
[tex]13.5=65[/tex] → [tex](20-13).9=63[/tex] → [tex]65+63=128[/tex]