Alan

Total protein:  P = 5 oz/(soldier*day) * 600 soldiers * 5*7 days = 105,000 oz

105,000 oz = 3 oz/(soldier*day) * N soldiers * 10*7 days     Solve for N

The force is inversely proportional to the distance squared, so:

A)  F2/F1 = 1/(2/5)^2 = (5/2)^2 = 25/4 = 6.25

B) F2/F1 = 1/7^2 = ...

The following graphs should help:

By inspection:

$\frac{1^3+6}{1}=7$

$\frac{2^3+6}{2}=7$

Unfortunately  $\frac{3^3+6}{3}\ne7$ 

However $\frac{(-3)^3+6}{-3}=7$

I'll leave you to finish.

Plot the left hand side against x, then plot the right hand side against x and see where they intersect.  You can do this on the online Desmos graphing calculator.

D = x - 4

R = D + 2x

S = R - 5

R + 2S + 3D = D + 2x + 2(D + 2x - 5) + 3D   =  6D + 6x - 10 = ... can you finish from here?

For your other equation you need to do some manipulation first:

results in

You can do the following  (notice the x=x at the end):

 to get:

I've no idea why this works!!

Like so:

Let S be the total number of students.  Then

0.2S + 42 = S

0.8S = 42

S = 42/0.8 = 52.5

Wow, the class has half a student!

Are you sure you didn't mean 40 students like to play soccer?

Tem certeza que não quis dizer que 40 alunos gostam de jogar futebol?