The answer is 69.048%.
Mass of potassium dichromate (K₂Cr₂O₇) is 294 g, which is a 100%.
Given mass of chromium is 52 g. So, the mass percent of chromium in the compound is 17.687%:
52 g : x% = 294 g : 100%
x = 52 ÷ 294 × 100 %
x = 17.687%
Given mass of potassium is 39 g. So, the mass percent of potassium in the compound is 13.265%:
39 g : x% = 294 g : 100%
x = 39 ÷ 294 × 100 %
x = 13.265%
Therefore, the mass percent of oxygen in the compound is 69.048%:
100% - 17.687% - 13.265% = 69.048%.