A sample containing beryllium (atomic mass 9 u), aluminum (27 u), and an unknown element is placed in a mass spectrometer. The ions all have the same charge and are accelerated through the same potential difference before entering the magnetic field. The beryllium and aluminum lines are separated by 2.550 cm on the photographic plate, and the unknown element makes a line between them that is 1.161 cm from the beryllium line.
What is the mass of the unknown element?