If y=7.5 when x=0.5, find y when x=-3My algebra one teacher is horrible! I never understand anything she is saying! Please explain in simple terms!

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ndnordic's profile pic

ndnordic | High School Teacher | (Level 2) Associate Educator

Posted on

In general you can set up probems like this as a ratio where y1/x1 = y2/x2.  In your problem, y1 is 7.5, x1 is 0.5 and x2 is -3.  y2 is what you want to find. You then have 7.5/0.5 = y/-0.3 and you can solve for y.  If you use this ratio methodyou can solve for any of the four variables by entering the known values for the other three.

pohnpei397's profile pic

pohnpei397 | College Teacher | (Level 3) Distinguished Educator

Posted on

Okay, the first thing you need to do is figure out what the relationship is between x and y.  Here's how you do that:

First, write a direct-variation formula

y = kx  where k is the constant of variation.  Plug in the y and x values that you know.

7.5 = .5k

To find for k, multiply both sides by 2 and you get k = 15

By that, we know that y is 15 times bigger than x.

So now all you have to do is plug your other value for X into that equation (y = kx).  Remember that we know that k = 15

So that gives us y = 15*-.3

And when you do the math, you get

y = -4.5

 

Wiggin42's profile pic

Wiggin42 | Student, Undergraduate | (Level 2) Valedictorian

Posted on

If y=7.5 when x=0.5, find y when x=-0.3

You can set up proportions: 

`(.5)/(7.5) = (-0.3)/(y)`

`.5y = -2.25`

`y = -2.25 /.5`

`y = -4.5`

``

` `

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