so
400 is the first cost
profit=x-400
40 per student (each student=1 hour)
x=students, since hour=1
40 times x
profit=40x-400
15 students per week
so
profit per week=40(15)-400
profit per week=600-400
profit per week=200
1 month is about 4 weeks so
200 times 4=800
they make 800 a month