A student balances a ruler on a pivot at the 15cm mark. The student places a cylindrical pencil sharpener with mass of 150g at the 25.0 mark and a 50.0g eraser at the 5.0cm mark while holding the ruler. The student has a second eraser with a mass of 80.0g and wants to balance the ruler and let go without everything falling. At what mark on the ruler must the student place the 80.0g eraser?