Question

Sal's Sandwich Shop sells wraps and sandwiches as part of its lunch specials. The profit on every sandwich is $2 and the profit on every wrap is $3. Sal made a profit of $1,470 from lunch specials last month. The equation 2x + 3y = 1,470 represents Sal's profits last month, where x is the number of sandwich lunch specials sold and y is the number of wrap lunch specials sold. Change the equation to slope-intercept form. Identify the slope and y-intercept of the equation. Be sure to show all your work. Describe how you would graph this line using the slope-intercept method. Be sure to write using complete sentences. Write the equation in function notation. Explain what the graph of the function represents. Be sure to use complete sentences. Graph the function. On the graph, make sure to label the intercepts. You may graph your equation by hand on a piece of paper and scan your work or you may use graphing technology. Suppose Sal's total profit on lunch specials for the next month is $1,593. The profit amounts are the same: $2 for each sandwich and $3 for each wrap. In a paragraph of at least three complete sentences, explain how the graphs of the functions for the two months are similar and how they are different. Below is a graph that represents the total profits for a third month. Write the equation of the line that represents this graph. Show your work or explain how you determined the equations.

Answer 1

Slope-intercept form is y = mx + b, where m is the slope and b is the y-intercept.  To write this in slope-intercept form we must isolate the y:
2x + 3y = 1470
2x + 3y - 2x = 1470 - 2x (subtraction will cancel the positive 2x on the left side of the equation)
3y = -2x + 1470 (since they are not like terms we cannot combine them, we leave them separate)
3y/3 = -2/3x + 1470/3 (cancel the 3 by dividing; EVERYTHING gets divided to keep it equal)
y = -2/3x + 490
The slope of this equation is -2/3 and the y-intercept is 490.
To graph this equation, plot 490 on the y-axis first, since it is the intercept.  Then count over to the right 3 and down 2 to find the next point; continue this for all successive points.
In function notation this would be f(x) = -2/3x + 490.  This function shows how the profit on wrap specials changes as the number of sandwich specials sold increases.  The graph of the function is attached.
The next month, when Sal's profit increased, the function changes because the y-intercept changes.  The slope stays the same.