Solution:
Since x varies directly as y, consider the following diagram:
by cross-multiplication, we get:
[tex]5x\text{ = (9)(10)}[/tex]
this is equivalent to:
[tex]5x\text{ = 90}[/tex]
solving for x, we get:
[tex]x\text{ = }\frac{90}{5}=18[/tex]
so that, we can conclude that the correct answer is:
x = 18.
