Answer:
11 hours
Step-by-step explanation:
Let's say Karen makes x hours babysitting and y hours giving music lessons.
Because she worked a total of 18 hours, we know that x + y = 18.
5 * x is how much money she earns babysitting for x hours, while 12 * y is how much money she earns giving music lessons for y hours. Adding them up gives her total revenue:
5x + 12y = 139
We now have a system of equations. Let's multiply both sides of x + y = 18 by 12:
12 * (x + y) = 18 * 12
12x + 12y = 216
Now, subtract 5x + 12y = 139 from 12x + 12y = 216:
12x + 12y = 216
- 5x + 12y = 139
_________________
7x + 0y = 77
Divide both sides by 7:
x = 11
Thus, Karen spent 11 hours babysitting.
