y varies directly with x if and only if there exists a constant k such that
[tex]y=kx[/tex]
for every input/output couple. If so, we have
[tex]\dfrac{y}{x}=k[/tex]
Let's see if our input/output couples satisfy this request: the first couple yields
[tex]\dfrac{y}{x} = \dfrac{6.4}{4}=1.6[/tex]
The second couple yields
[tex]\dfrac{y}{x} = \dfrac{11.2}{7}=1.6[/tex]
The third couple yields
[tex]\dfrac{y}{x} = \dfrac{16}{10}=1.6[/tex]
The last couple yields
[tex]\dfrac{y}{x} = \dfrac{20.8}{13}=1.6[/tex]
So, the ratio is indeed constant
