If 800 left and nobody had been replaced then they would only have 11,200 at the end of the year. They actually had 11,500 at the end of the year which means that 11,500 – 11,200 = 300 had been replaced.
Therefore the turnover ratio is 300 divided by the average number of employees (i.e. divided by (12,000 + 11,500)/2 )