In this problem the terms have different powers, and so can not be added directly. Instead one term must be converted

2.55 x 10^-7 + 6 x 10^-8 = 2.55 x 10^-7 + 0.6 x 10^-7 = (2.55 + 0.6) x 10^-7 = 3.15 x 10^-7