Your picture shows 3 combinations that are sufficient to tell you the price of each item.
The other two are
.. 0 fries + 1 shake + 1 burger = 4.20
.. 1 fries + 1 shake + 0 burger = 2.80
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Add any two orders and subtract the third to find the cost of one of the items.
For example add the two orders written above and subtract the one shown in the picture.
.. (1 fries + 2 shakes + 1 burger) - (1 fries + 1 burger) = (4.20 +2.80) -(3.40)
.. 2 shakes = 3.60
.. 1 shake = 1.80
Then the last equation tells you
.. 1 fries = 2.80 - 1 shake = 1.00
and the first equation tells you
.. 1 burger = 3.40 - 1 fries = 2.40
1 fries = $1.00
1 shake = $1.80
1 burger = $2.40
