The rule is, for a mixed number represented by a+b/c, the improper fraction is found by using the formula (ac+b)/c. This means we take the denominator of the fraction part and multiply it by the whole number part then add the numerator of the fraction part, this becomes the numerator of the improper fraction, and the denominator of the improper fraction is the same as the numerator of the fraction part.
So in this case we have a=8, b=3 and c=8, so the improper fraction is (8×8+3)/8=67/8.