Let Karen works x hours baby sitting
So, she worked (18-x) hours giving music lessons
So,
According to question
5x + 12(18-x)=139
This gives
7*x=77
x=11
So,
<span>Karen worked <em>11 hours baby sitting </em>and (18-11)=7 hours giving music lessons</span>
The answer is 4 pensioners
Add 8 to both sides
3x=33
Divide both sides by 3
x=11