Question

After the booster club sold 40 hotdogs at a football game, it had 90$ in profit. After the next game, it had sold a total of 80 hotdogs and a total of $210 in profit. Which equation models the total profit, y, based on the number of hotdogs sold, x?

Answer 1

Let's first establish what we already know for this problem.

x = total number of hotdogs sold
y = total profit from total sales of hotdogs

Let's also establish the other equations which we will require in order to solve this problem.

Equation No. 1 -
Profit for 40 hotdogs = $90 profit

Equation No. 2 -
Profit for 80 hotdogs = $210 profit

STEP-BY-STEP SOLUTION

From this, we can use the formula y = mx + b & substitute the values for x & y from one of the two previous equations into the formula in order to obtain the values of m & b for the final equation. Here is an example of the working out as displayed below:

Firstly, using the first or second equation, we make either m or b the subject. Here I have used the first equation and made m the subject:

Equation No. 1 -
y = mx + b
90 = m ( 40 ) + b
40m = 90 - b
m = ( 90 - b ) / 40

Now, make b the subject in the second equation as displayed below:

Equation No. 2 -
y = mx + b
210 = m ( 80 ) + b
210 = 80m + b
b = 210 - 80m

Then, substitute m from the first equation into the second equation.

Equation No. 2 -
b = 210 - 80m
b = 210 - 80 [ ( 90 - b ) / 40 ]
b = 210 - [ 80 ( 90 - b ) / 40 ]
b = 210 - 2 ( 90 - b )
b = 210 - 180 - 2b
b - 2b = 30
- b = 30
b = - 30

Now, substitute b from the second equation into the first equation.

Equation No. 1 -
m = ( 90 - b ) / 40
m = ( 90 - ( - 30 ) / 40
m = ( 90 + 30 ) / 40
m = 120 / 40
m = 3

Through this, we have established that:

m = 3
b = - 30

Therefore, the final equation to model the final profit, y, based on the number of hotdogs sold, x, is as follows:

y = mx + b
y = ( 3 )x + ( - 30 )

ANSWER:
y = 3x - 30