We know that f(x) varies directly with x, this means that the function can be written as:
[tex]f(x)=kx[/tex]we also know that when f(x)=90 then x=50, then we have:
[tex]\begin{gathered} 90=50x \\ x=\frac{90}{50} \\ x=\frac{9}{5} \end{gathered}[/tex]Hence the function is:
[tex]f(x)=\frac{9}{5}x[/tex]is x=10 then we have:
[tex]f(10)=\frac{9}{5}(10)=18[/tex]Therefore the value of f(x) is 18 when x=10
