The constant of proportionality can be find as:
[tex] \frac{y}{x} = k[/tex]
From the table, we can get y divided by x as:
[tex] \frac{90}{9} = 10 [/tex]
[tex] \frac{20}{2} = 10 [/tex]
[tex] \frac{50}{5} = 10 [/tex]
[tex] \frac{30}{3} = 10 [/tex]
[tex] \frac{70}{7} = 10 [/tex]
Then, it is obvious that:
[tex]k = 10[/tex]
Lastly:
[tex]y = 10x[/tex]
