Eric V.

asked • 12/16/14

The variables x and y are directly proportional, and y=4 when x=3. What is the value of y when x=15?

algebra problem help!

Tiami B.

Since the variable x and y are directly proportional, they are related by an equation y=kx, where k is a constant. 
 
y=kx
4=k(3)
4/3=k
 
Therefore y=4/3, and so when x=15, the value of x is y= 4/3(15) or 20
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10/28/16

1 Expert Answer

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Charity T. answered • 12/16/14

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K-12, Math (PreAlgebra, Algebra, Geometry, PreCal, Calculus)

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Hi Eric. 
Directly proportional means that when y increases, x is also increasing. The basic formula you'd be able to use is y=mx
You can then use your given points (3,4) to find out what m is.
4=3m by simply pugging it into the equation.
 
 
So m is 4/3. Now you have your equation of y=(4/3)x.
next you want the value of y when x is 15 so plug 15 into x for your equation and you'll be able to find y.
ill leave that part up to you. :) feel free to let me know if you have any question
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