Question 1155672
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Suppose that today, instead of 3 burgers and 1 fries for $13.80, he had bought twice as much -- 6 burgers and 2 fries for $27.60.<br>
Then the difference between yesterday's purchase and today's would be 1 more burger for $3.95.<br>
So the price of a burger is $3.95; from there it is easy to find the price of an order of fries.<br>
That informal solution using logical reasoning follows exactly the same path as a formal algebraic solution using elimination:<br>
{{{5h+2f = 23.65}}}  -- (1) yesterday
{{{3h+f = 13.60}}}  --  (2) today
{{{6h+2f = 27.60}}}  --  (3) today's, doubled
{{{h = 3.95}}}  --  the difference between (1) and (3)<br>