Question

What is the weight of the 2 liter jug?

Answer 1

You can do this:
5/7.5=2/x 
cross multiply, and get 5x=15
x=3
Thus, the weight of a two-liter jug should equal 3kg
Hope this helps!

Answer 2

$\bf \begin{array}{ccll} liters&kgs\\ \text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\ 5&7.5\\ 2&k \end{array}\implies \cfrac{5}{2}=\cfrac{7.5}{k}\implies k=\cfrac{2\cdot 7.5}{5}$