Chris is a college student who can work a maximum of 40 hours per week. he needs to make at least $900 in order to cover his expenses. his tutoring job pays $25 per hour and he also makes $15 per hour refereeing soccer matches. find the constraints for this situation and determine which point is a valid solution to the system, where x is the number of hours chris spends tutoring and y is the number of hours he spends refereeing.