Haemoglobin contains 0.25% of iron by eight. If molecular weight of haemoglobin is 89600, how many iron atoms are present in each haemoglobin molecules ?

Mass of iron in one mole of Haemoglobin = 

$\frac{89600\times 0.25}{100}$  =  224 g/mol.
As we know, molar mass of iron is 56 g/mol. 
So, number of iron atoms in one mole of Haemoglobin  =$\frac{224}{56}$ = 4.