Hey Ashish, 

Atomic Number is equal to Number of Protons and electrons in an atom. And Atomic mass or Mass number are equal to sum of Number of neutrons and number of Protons in an atom. 

So, Number of Protons /Electrons = 25

Number of Neutrons = 30

Therefore, the nucleus of the atom has 25 Protons and 30 Neutrons in it.

Feel free to ask any other doubt! 

Good Luck !