suppose 4.52 g H2o and 20.09 g So3 are produced. How many grams of H2So4 decompose?

Wouldn't the answer be 24.61?

H2SO4 ==> H2O + SO3

...x......4.52 + 20.09
x = 4.52 + 20.09 = 24.61

Law of Conservation of mass. Starting mass = ending mass.