with(Tolerances):

a := `&+-`(2, .5);

b := `&+-`(30, 10);

evalf(a*b);

with(ScientificErrorAnalysis):

a1 := Quantity(2, .5);

b1 := Quantity(30, 10);

combine(a1*b1, errors);

Dear everyone, 

       I want to deal with experimental data with error. I find two ways to do the job. But  the results are different .Could you tell me which one is more correct?

      Thank you.

Barry.