There is a special kind of substitution which some books call it by comparison.
When the two equations are both y=some function of x y=some other function of x, then we can substitute the second equation into the first giving some function of x = some other function of x and start solving.
For example, <span>y = –2x + 8 y = x – 1 substitute second into first x-1=-2x+8 isolate x on left, 3x=9 x=3 second step is to substitute x=3 into second equation to get y=x-1=3-1=2 Therefore the solution is (3,2)</span>
<span>valid. She used a representative sample </span>because she correctly found the average and 18,000 is a reasonable number to get for a the problem
