What percent of magnesium bromide, mgbr2, is magnesium?

First, we need to calculate the molar mass of the whole compound by adding the masses of each atom from the periodic table. 

Mg= 24.3 g/mol
Br= 80.0 g/mol

molar mass MgBr2= 24.3 + 2 x 80.0= 184.3 g/mol

to find the percent of magnesium doing "parts over the whole"
the mass of magnesium over the total mass. then times 100 because it is a percentage value

24.3/184.3x 100= 13.2%

Most likely a is the answer - because  you generate   38 atp's but also consume an atp.   each atp gives approx. less than 4 kcal so probably a is the closest answer.