A cobalt (atomic no. = 27) target is bombarded with electrons, and the wavelengths of its characteristic X-ray spectrum are measured. A second weak characteristic spectrum is also found, due to an impurity in the target. The wavelengths of the Kα lines are 225.0 pm (cobalt) and 100.0 pm (impurity). If the atomic number of the impurity is 10 N. The value of N is _________(take b = 1)