An atom has 7 electrons in its M - shell and contains 18 neutrons in its nucleus. What is its mass number

According to Bohr model, Each shell can occupy 2 $n2$ electrons in their orbits where n=1,2,3.. for K,L,M,..

The atom has 2 electrons in k, 8 electrons in L and 7 electrons in M = 2 + 8 + 7 = 17

And given that its nucleus has 18 neutrons

=> its mass number = number of (electrons + neutrons ) = 17 + 18 = 35