SOLUTION
The weight(w) of an object varies directly with the mass(m) of the object.
[tex]w\alpha m[/tex]Introducing a constant k
[tex]w=km[/tex]Given
[tex]\begin{gathered} w=12N \\ m=4kg \end{gathered}[/tex]Substitute the values of the weight and mass in order to solve for k
[tex]\begin{gathered} 12=4k \\ k=\frac{12}{4}=3 \\ \therefore k=3 \end{gathered}[/tex]Hence, the equation connecting the weight and mass is
[tex]w=3m[/tex]Let us now solve for the mass given the weight to be 21N
[tex]\begin{gathered} 21=3m \\ m=\frac{21}{3}=7 \\ \therefore m=7kg \end{gathered}[/tex]Hence, the answer is 7kg.
