Represent the empirical formula by xK+yCr+zO, where x, y and z are whole numbers.
Now replace the element symbols by their approximate atomic weights:
39x+52y+16z and call this value M.
39x/M=0.2657, so x/M=0.2657/39=0.006813 approx.
Similarly, y/M=0.3536/52=0.0068, so x:y=1:1.
z/M=0.3807/16=0.024 so z:x=0.024/0.0068=3.5 approx.
So we have 1:1:3.5=2:2:7=Cr₂K₂O₇ or K₂Cr₂O₇ as the empirical formula.