0.80 g of sample of impure potassium dichromate was dissolved in water and made up to 500 mL solution 25 mL of this solution treated with excess of KI in acidic medium and I2 liberated required 24 mL of a sodium thiosulphate solution. 30 mL of this sodium thiosulphate solution required 15 mL of N/20 solution of pure potassium dichromate. What was the percentage of K₂Cr₂O₇ in given sample ?