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Suppose y varies directly with x, and y=15 when x=-5. Find x when y= 18.

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Answer:

x = 6 when y = 18

Explanation:

"y varies directly with x" is represented by y ∝ x and translates to the equation y = kx

where k is a constant known as the constant of proportionality

Given y = 15, when x =5 we can determine k
15 = k · 5

k = 3

So the proportionality equation is

y = 3x

When y = 18,

18 = 3x

x = 18/3 = 6


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