Question

An atom contains electrons protons and neutron if the mass of each neutrons is halved and each electron is doubled then the electron is doubled then the atomic mass of 12Mg24

Answer 1

In the formula been provided 
12 is the atomic mass 
atomic mass = No of proton 

24 is the mass number 
mass number = mass of protons and neutrons

the mass of electron is negligible in this. so if the mass of neutron is halved than it will be 6
than the atomic mass will be = 12 + 6 = 18

Answer 2

In 12mg24 

12 is the atomic number that is number of protons 

24 is the mass number. It is 12+12 ( 12 of protons and 12 of neutrons) 

Mass of proton ≈  mass of neutron 

Mass of electrons is negligible 

Even doubling it does not affect the atomic mass significantly  

If mass of neutron  is halved, the mass  of 12 neutrons = 6 

Then atomic mass will be 12 (proton mass)  + 6(neutron mass)  = 18