"you start with 15.67 g mgco3-xh2o, when heated and all the water is removed, you are left with 7.58 g mgco3 only.
therefore, you had 15.67 - 7.58 = 8.09g h2o
moles of each :
7.58g mgco3 = ? moles = 7.58g / 83.4g/mole = 0.09moles
8 09g h2o = ? moles = 8.09g / 18g/mole = 0.44moles
molar ratio between the two by dividing both numbers by the smallest number :
0.09 / 0.09 = 1
0.44 / 0.09 = 4.8