We have the relationship between two variables that can be modeled by a line equation. We need first to determine the equation of the line. To do that can use the following equation for a line:
![y=mx+b](https://img.qammunity.org/2023/formulas/mathematics/high-school/smsb8cbft03lwblmi49nf2l6jby2ofxzws.png)
Where "m" is the slope and b the y-intercept. To determine the slope "m" we use the following formula:
![m=(y_2-y_1)/(x_2-x_1)](https://img.qammunity.org/2023/formulas/mathematics/high-school/78uaqhwt0aws3qfwxigaftpihnmb1gzxtp.png)
Taking two points in the line:
![\begin{gathered} (x_1,y_1)=(2,246) \\ (x_2,y_2)=(4,450) \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/high-school/u532nb21voh7tw1odmto74t8pg2z9mbv75.png)
Replacing in the formula for the slope:
![m=(450-246)/(4-2)](https://img.qammunity.org/2023/formulas/mathematics/high-school/uqghhfn2gm1zwqs3xhhkhhtab3lj7gphxi.png)
Solving the operations:
![m=(204)/(2)=102](https://img.qammunity.org/2023/formulas/mathematics/high-school/3euy9llum7vm9q7zrjhfrfw2zocah5zsqy.png)
replacing in the equation for the line:
![y=102x+b](https://img.qammunity.org/2023/formulas/mathematics/high-school/vlz43lnh7dan3orgbi0ijgpbhn9pvk6mds.png)
To find the value of "b" we replace any point through which the line passes. Replacing (x,y) = (2,246):
![246=102(2)+b](https://img.qammunity.org/2023/formulas/mathematics/high-school/jnqs6xf1eo9ooaepdjm51rnqhqkojfwguo.png)
Solving the product:
![246=204+b](https://img.qammunity.org/2023/formulas/mathematics/high-school/zl7nniohiw0n2dev620gbrkl7jn2zz1w1b.png)
subtracting 204 to both sides:
![\begin{gathered} 246-204=b \\ 42=b \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/high-school/qu0r4ltwrpfbv7teqwcvigzermy5iffjzt.png)
Replacing in the line equation:
![y=102x+42](https://img.qammunity.org/2023/formulas/mathematics/high-school/2mbd9yy7mvmevjtny56bd3nz673loy1w8x.png)
The processing fee would be the initial fee and in the line equation, it is equivalent to the y-intercept therefore, the processing fee is $42. The daily fee is equivalent to the slope of the line, therefore, the daily fee is $102 per day.
If the total amount to spend is $1200 then replacing in the line equation:
![1200=102x+42](https://img.qammunity.org/2023/formulas/mathematics/high-school/sxz26fmrrvc5tctk41v7mgqum8rjvkagsb.png)
Solving for "x" first by subtracting 42 to both sides:
![\begin{gathered} 1200-42=102x \\ 1158=102x \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/high-school/gv5aa7l0qx5rw9n88zhzpbf0bgqbaw0c5u.png)
Dividing both sides by 102:
![\begin{gathered} (1158)/(102)=x \\ 11.4=x \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/high-school/80dsrhbx5no21d6ij449z6o1rrjxznm10f.png)
Therefore, the number of days is 11.4.