Question 73111
Notice how c=4p and s=p-2, you can simply replace c and s in the first equation with 4p and p-2. This allows you to work with nothing but p. 
{{{s + p + c = 40}}}
{{{c = 4p}}}
{{{s = p - 2}}}
{{{s + p + highlight(4p) = 40}}}Replace c with 4p (it's like saying x+5=10 and if I replace x with 5 I get 5+5=10. What I'm doing is substituting 5 into x to show that they are equal. In this case I'm working with p's and c's).
{{{highlight(p-2) + p + highlight(4p) = 40}}}Do the same thing to s and replace s with p-2
{{{(p-2)+p+4p=40}}}Now we have gone from 3 variables to 1 variable. We can now solve for p.
{{{p+p+4p-2=40}}}Group like terms
{{{6p+cross(-2+2)=40+2}}}Add like terms and add 2 to both sides
{{{cross(6/6)p=42/6}}}Divide both sides by 6
{{{p=7}}}
Now that we have p=7 we can use this to solve for c and s
{{{c=4p}}}Solving for c
{{{c=4(7)}}}
{{{c=28}}}
{{{s=p-2}}}Solving for s
{{{s=7-2}}}
{{{s=5}}}
So p=7, s=5, and c=28
<p>
Check:
{{{s + p + c = 40}}} Use the first equation
{{{5 + 7 + 28 = 40}}}
{{{12+28=40}}}
{{{40=40}}}Works
<p>
{{{c = 4p}}}Use the 2nd equation
{{{28 = 4(7)}}}
{{{28 = 28}}}Works
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{{{s = p - 2}}}Check with the 3rd equation
{{{5=7-2}}}
{{{5=5}}}Works
Hope this helps.