Question

If x varies directly as $3y+1$ and $x=9$ when $y=1$, then what is the value of x when $y=5$?

• A. $11$
• B. $10$
• C. $20$
• D. $36$

Answer 1

The correct option is D $36$

Given that, $x$ varies directly as $3y+1$

$\therefore x=k(3y+1)$ , $k$ is some constant

at $y=1,x=9$

$k\left(3\right(1)+1)=9$

$k=\frac{9}{4}$

$x=\frac{9}{4}(3y+1)$

at $y=5,x=\frac{9}{4}\left(3\right(5)+1)=\frac{9}{4}\times 16$

$\therefore x=36$