The mass of a jar $\dfrac{1}{4}$~full of Marmite is $$250\text{g}$$. It has a mass of $$350\text{g}$$ when it is $\dfrac{3}{4}$full. What is the mass of the empty jar? 
Overseas Competition->Knowledge Point->Word Problem Modules->Solving Problems Involving Fractions and Percentages->Finding the Base