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Answer:
when it doesn't fall on a grid intersection
Step-by-step explanation:
Graphical solutions work nicely when the solutions fall squarely on points that are intersections of the graphical grid. Sometimes, off-grid solutions can be found exactly when they are recognizable fractions, such as 1/2 or 2/3. Otherwise, it can be difficult to determine an exact solution from a graph.
An estimate is needed when the exact solution is not a grid point.
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The attached graph shows two systems of equations. In one case, the solution is (1, 1), on a grid point. In the other case, the solution is approximately (0.308, 0.308). (It is actually (4/13, 4/13), where 4/13 is the repeating decimal fraction 0.307692....)
