We know that
• 1lb = 0.4536 kg.
,• 1ft^3 = 0.0283 m^3.
Now, we can write an expression that helps to make the unit's transformation.
[tex]x=\frac{80lb}{ft^3}\cdot\frac{0.4536\operatorname{kg}}{1lb}\cdot\frac{1ft^3}{0.0283m^3}[/tex]Observe that the same magnitudes we write them down in opposite positions, that way, they will be simplified. Let's solve
[tex]\begin{gathered} x=\frac{36.288}{0.0283}kg/m^3 \\ x\approx1,282.26kg/m^3 \end{gathered}[/tex]