Answer:
17 days
Explanation:
For each day she works, she earns +55 and each day she DOES NOT work she earns -66. Total 30 days.
Let number of days she works be x
thus, number of days she DOES NOT work is 30 - x
We can setup an equation as:
55(x) + -66(30-x) = 77
This means, she works x days for 55 each and 30 - x days getting -66 each, totalling 77.
We can solve for x to find number of days she worked. Work shown below:

Thus, she worked 17 days