Lisa gives child(x) and adult(y) haircuts in her salon. She charges $10 for a child and $40 for an adult haircut. Her goal is to make at least $1000 per week and work no more than 40 hours. The shaded region of the graph represents the possible combinations of adult and child haircuts that will allow her to meet her goal. If Lisa gives 60 child haircuts she must give _____ adult haircuts to meet her goal.
A: 10
B: 15
C: 20
D: 25