We can use the following formula in order to solve all the questions:
[tex]\begin{gathered} \rho=\frac{m}{v} \\ \text{where:} \\ \rho=Density \\ m=mass \\ v=volume \end{gathered}[/tex]2)
[tex]\begin{gathered} v=\frac{m}{\rho} \\ m=1\operatorname{kg}=1000g \\ \rho=\frac{13.6g}{ml} \\ v=\frac{1250}{17}ml\approx73.53ml \end{gathered}[/tex]3)
[tex]\begin{gathered} v=\frac{m}{\rho} \\ m=275.7g \\ \rho=\frac{7.1g}{ml} \\ v=\frac{275.7}{7.1} \\ v=\frac{2757}{71}ml\approx38.83ml \end{gathered}[/tex]