Question

Find an equation of variation where y varies directly as x. One pair of values is y = 80 when x = 40.

Answer 1

y=kx
80=40k
divide by 40 on both sides
2=k
so the equation is y=2x

Answer 2

\(\normalsize\color{blue}{ \rm y=kx }\) where \(\normalsize\color{blue}{ \rm k }\) is a constant. Plug in your pair of values to find k.
\(\normalsize\color{blue}{ \rm y=kx }\)
\(\normalsize\color{blue}{ \rm 80=k(40) }\)
\(\normalsize\color{blue}{ \rm 2=k }\)
Thus, you get \(\normalsize\color{blue}{ \rm y=2x }\) .

Answer 3

Thankkss @acxbox22 & @SolomonZelman