An ion with mass number 56 contains 3 units of positive charge and 30.4% more neutrons than electrons will have  Z as

 

Mass number (N + P) = 56
Let Protons be = P
Then, neutrons = (56 - P)
$$\begin{gathered} \mathrm{and} \mathrm{electrons} \mathrm{in} {\mathrm{x}}^{3+}=(\mathrm{P}-3) \\ \mathrm{Neutrons} =(\mathrm{P}-3)\frac{30.40}{100}+(\mathrm{P}-3) \\ =(\mathrm{P}-3)1.3040 \\ \therefore \mathrm{mass} \mathrm{number}=\mathrm{no}.\mathrm{of} \mathrm{p}+\mathrm{no}.\mathrm{of} \mathrm{n} \\ 56=\mathrm{P}+(\mathrm{P}-3)1.304 \\ (\mathrm{P}-3)1.3040=56-\mathrm{P} \\ 56=2.304\mathrm{P}-3.912 \\ 59.912=2.304 \mathrm{P} \\ \mathrm{P}=26=\mathrm{Z}\mathrm{ } \end{gathered}$$