Question 840818
d for count of disks
v for count of videos


Account for the count:  d+v=40
Account for sales revenue:  4d+6v=180


The revenue equation should be simplified, dividing left and right by 2:
2d+3v=90


Elimination Method on THIS system would be most convenient:
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2d+3v=90
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d+v=40
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...
{{{highlight(v=10)}}}, done in head; I used multiplication of the "40" equation by 2 and subtracted from the "90" equation.
{{{highlight(d=30)}}}