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In direct variation, what y varies directly as x means that y/x=k. k is a constant

Therefore, with y/x said, in the problem y=3, x=15:

It will be set up as:

3/15=k

This is because we plug the y's in in the place of the 'y' and the x's in in place of the 'x'.

Now, there is your next equation:

Find y when x=-25.

Here, you may use a variable.

It may be set up as:

y/-25=k

Since these two expressions are equal to the same constant, k, we may set them equal to each other:

3/15 = y/-25

By cross multiplication, we get that this equation is equal to:

3*-25 = 15*y
-75 = 15y
-5 = y

Your answer is -5

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You did not ask this question but direct variation goes hand in hand with this.

This is Inverse Variation:

It is usually said like this:

y varies inversely as x.

This basically means: k = yx rather than k = y/x

This is Joint Variation.

Joint Variation is a mix between inverse and direct variation.

It is usually said like this:

y varies directly as x and inversely as z

This means:

k = yz/x


P.S. I will put this in the mathematics formulae sticky.