Let us consider a volume of 1L `MgCl_2` solution.
Mass of the solution `= 1000xx1.35 = 1350g`
It is given that the mass percentage of `MgCl_2` is 55.5%
Mass of `MgCl_2 = 1350xx55.5% = 749.25g`
Molecular mass of `MgCl_2 = 95.211g/mol`
Number of `MgCl_2 ` moles in the solution `= 749.25/95.211 = 7.87mol`
So we have 7.87moles in 1L solution.
Concentration of `MgCl_2` is 7.87M.