Calculate the number of moles of magnesium, chlorine, and oxygen atoms in 7.80 moles of magnesium perchlorate, mg(clo4)2.

 The number of moles of moles of  Magnesium,chlorine and oxygen atoms in 7.80 moles of Mg(ClO4)2 is calculated as below

find the total number of each atom in Mg(ClO4)2

that is mg = 1 atom
        Cl =  1x2 = 2 atoms
        O  = 4  x2 = 8 atoms

then multiply 7.80 moles with total number of each atom , to get the number moles of each atom
that is

Mg = 7.80 x1= 7.80  moles
cl =   7.80  x2=15.6  moles
O =  7.80 x8= 62.4 moles

Do u have the n? or maybe the m?