If y varies directly with x, and If y = -16 when x = 4, how do you find x when y = 20?

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Answer 1 · 2016-03-02T17:09:36

When $y =20$ , $x$ is $color(Blue)(-5$

$y$ varies directly with $x$ this implies $y$ is directly proportional to $x$ :
$color(blue)(y prop x$

Upon introducing constant $k$ (constant of proportionality ):
$y = color(green)(k) x$ ........ $(1)$

As per given data: when $y=-16, x=4$ , substituting the above values in $(1)$ to find $color(green)(k$

$-16 = k xx 4$

$k = -16/ 4$

$color(green)(k = -4$ ,

Finding $x$ when $color(blue)(y =20$ :

$y=kx$

$color(blue)(20) = color(green)(-4) xx x$

$x = 20/ (-4)$

$color(blue)(x= -5$