Last one, with math 18-20 on the pdf. i do not know how to post a picture i am so sorry.

Question 18
The answer is 4 days. 
Because, 
Kevin =3×n
Anita =4+ (2×n)
Equating the two equations. 
3n=4+2n
3n-2n=4
n=4

Question 19
When the solution is (4,1), it means the value of y=1 and x=4. 
The answer is;
y=x-3
y=-3x+13. Since when the values of x and y are substituted they satisfy the system. 

Question 20
The system has no solution.
This is because the line are parallel and they do not intersect to give a solution. 

Hello,
Please, see the attached files.
Thanks.