Question

An empty 1000 mL container has a mass of 250 g when filled with a liquid that container in the liquid have a combined mass of 1300 g

Answer 1

Total Weight - Container weight = Liquid Weight

1300 - 250 = 1050 / 1.05 = 1000 (amount filled in the container)

So
1050 g

The weight of each g/ml must be 1.05 in order to get 1050 ml.

The answer is 1.05

( Credit given to user "Aakasha A.', information above used only to pass on information that has been answered already )