You have 100 +/- 10 and 80 +/- 8. When calculating the difference of two quantities the errors would typically be "added in quadrature", so the error is the square root of 10 squared plus 8 squared, i.e. root 164 or 12.8. The difference is thus 20 +/- 12.8 and 12.8 is 64% of 20.