A public school currently it pays all teachers same, regardless of their area of expertise. At $70,000, it appoints good teachers in all areas except science. Because science is the field in demand, the school can only appoint poor science teachers. It would have to pay science teachers $90,000 to appoint good science teachers. If one out of five teachers is the science teacher, determine the marginal expense of getting a good science teacher instead of a poor one?