Answer:
F) 11
Step-by-step explanation:
We know that she worked 18 hours, and earned $139.
Let's just say that she spent x hours baby-sitting.
If she makes $5 per hour, the total amount of money she earned from baby-sitting would be 5x.
If she spent x hours baby-sitting, that would mean that she spent 18-x hours giving music lessons.
So, the total amount of money she earned from music lessons would be 12(18-x).
Now we can plug this into an equation.
5x + 12(18-x) = 139.
5x + 216 - 12x = 139.
5x - 12x = -77
-7x = -77
x = 11.
This means that she spent 11 hours baby-sitting.
Hope this helps!