commercial sample of cyclohexane freezes at 6 degrees celcius.if freezing point of pure cyclohexane is 6.5 degree celcius and its constant is 20 deg per molal then calculate purity of sample

The formula related to freezing point depression and and molality is given as:
∆T = K_f x m
​​∆T = depression in freezing point = 6.5C - ​6C = 0.5C 
K_f = freezing point depression constant = 20$°$C mol^-1
m = molality of solution = ​∆T/ ​K_f  = ​0.5$°$C / ​20$°$C mol^-1=0.025 mol
0.025 moles of solute in 1000g of solvent. Here solvent is cyclohexane.
Molar mass of cyclohexane = 84 g mol^-1.
Number of moles of cyclohexane = 1000 g /84g mol^-1 = 11.9 mol
total number of moles = 11.9 mol + 0.025 mol =   11.925 moles
% purity = moles of cyclohexane /total number of moles x 100 = 11.9 /11.925 x 100 = 99. 7 % pure.