Question 622248
Hi, there--
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Let B be the number of burgers purchased.
Let F be the number of fries purchased.
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Burgers cost $3.00 times the number of burgers purchased, or 3.00B.
Fries cost $1.50 times the number of fries purchased, or 1.50F.
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Together the cost of the burgers plus the cost of the fries is $12.00, so
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{{{3.00B+1.50F=12.00}}}
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To graph this equation, I would translate it to slope-intercept form (y=mx+b). Let's write it in an "F=" format. Subtract 3.00B from both sides of the equation to isolate the F-term.
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{{{1.50F=12.00-3.00B}}}
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Divide both sides of the equation by 1.50.
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{{{(1.50/1.50)F=12.00/1.50-(3.00/1.50)B}}}
{{{F=8-2B}}}
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Rearrange the terms on the right side of the equation to get the y=mx+b form. Do this carefully, the coefficient of B is -2.
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{{{F=-2B+8}}}
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The slope of this equation is -2 and the y-intercept is 8. The graph of the equation is located below.
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{{{
graph( 300, 200, -1, 10, -1, 10, -2x+8 )
}}}
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My graph shows a little bit of the line going out of Quadrant I (the upper right hand corner). Actually the line stops at the points (4,0) and (0,8) because you cannot purchase a negative number of burgers or fries.
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We can use the graph to find three different combinations of burgers and fries totaling $12.00.We have two combinations already.
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(4,0) means order 4 burgers and no fries. Burgers cost $3.00 so you've spent all your money on burgers.
(0,8) means order 0 burgers and 8 fries. Fries cost $1.50 each, so you've spent all you money on fries.
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Any other combination of burgers and fries totaling $12.00 will be found on the line. 
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(NOTE: To be completely accurate, the graph should be a series of dots rather than a straight line. because you can only purchase whole burgers and fries. The algebra.com software doesn't have an easy way to graph that.)
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I'll leave out to you to use the graph to find a third combination. For example, if you bought two burgers, how many fries could you buy? Find that on the graph.
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Hope this helps! Email me if there is any part that I need to explain better.
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Ms.Figgy
math.in.the.vortex@gmail.com