How do you get part (c)?
Part (a) Which statement best describes the numbers of protons and electrons in the penny when it is charged as indicated?
Electrons were added, and the number of electrons exceeds the
number of protons. ✔ Correct!

Part (b) How many electrons were transferred in order to create the charge on the penny?
Ne = 6.88 * 109
Ne = 6.880×109electrons ✔ Correct!

Part (c) By what percentage do the transferred electrons change the mass of the penny? (Express your result as a positive percentage for an increase, negative for a decrease.)