How do i do that i need thus questions pls me with it

We all take more time to solve a system of two or more equations than an equation with a single unknown.  So the following will avoid system of 2 equations for this problem... by pre-substitution.

R=number of red apples
80-R=number of green apples
Equation is: 
R/5 =3(80-R)
Cross multiply and solve for R
R=15(80-R)
R+15R=1200
R=1200/16=75
So Mr. Teo has 75 Red apples (and 5 green apples)

In all honesty, there really is no “work” per say to show. mainly it’s just using the process of elimination. the question is asking which domain(s) would be the most appropriate for the function c(n). n is the number of observed vehicles so right away, 1, 2, and 3 can be eliminated.why? 1, 2 start in the negatives and therefore it is impossible to observe negative cars. so, #4 would be the best answer to the given problem.