Please help?
A fluoride ion electrode is used to measure the fluoride concentration in a cup of tea. When immersed in a mixture of 25 cm^3 of tea and 25 cm^3 of an ionic-strength adjustment buffer, the electrode gave a reading of 98 mV. When 2.0 cm^3 of a 100 ppm fluoride solution was added to this mixture, the reading became 73 mV. Calculate the concentration of fluoride ions in the tea.