A student places an object on the 5.0 cm mark of the meter stick. He places a positive lens that he knows has a focal length of 15.0 cm at the 30.0 cm mark on the meter stick. He now places a negative lens of unknown focal length at the 60.0 cm mark. He finds that there is a real image focused on the screen at the 90.0 cm mark of the meter stick. From the data what is the focal length of the negative lens?