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Solve where two models meet

About 13 minutes · 3 reading + 10 practice and checks
Time includes reading, practice, and questions. Extra videos or further study are optional.
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A linear-model comparison with tables, graphs, slope and intercept interpretations, a solved intersection and a limitation note.

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What to include

  • Include input/output definitions and bounded domains.
  • Provide checked tables and labeled linear graphs.
  • Interpret rate and starting output with units.
  • Solve an intersection and verify both outputs.
  • Document a capacity-limited model and all changed-case checks.

Review and revise

Use these criteria to check your work and choose what to improve.

  • Method: applies the course-specific rules to the supplied cases with visible intermediate work.
  • Accuracy: results agree with the stated facts, units, boundaries and independent checks.
  • Reasoning: explains why the result follows and identifies a limitation or unresolved question.
  • Transfer: completes changed cases and revises the final project when a relevant assumption changes.
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Before you begin

The idea in everyday life

Original case: Container A has A(t)=8+4t L; B(t)=20+t L; both apply for 0≤t≤8 minutes.

The idea

An intersection is an input where two models predict the same output. Set their expressions equal, solve, then calculate the shared output and check whether the input belongs to both model domains.

Follow along

Solve where two models meet — worked practice

An intersection is an input where two models predict the same output. Set their expressions equal, solve, then calculate the shared output and check whether the input belongs to both model domains.

Original case: Container A has A(t)=8+4t L; B(t)=20+t L; both apply for 0≤t≤8 minutes.

Case information

Paste at A1.

ARecordBGiven
Original caseOriginal case: Container A has A(t)=8+4t L; B(t)=20+t L; both apply for 0≤t≤8 minutes.

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  1. 1. 1. Set equal outputs

    Write the equation for equal volume.

    You should see: 8+4t=20+t.

  2. 2. 2. Solve the input

    Subtract t and eight, then divide by three.

    You should see: 3t=12; t=4 minutes.

  3. 3. 3. Check both outputs

    Evaluate A(4) and B(4).

    You should see: Both contain 24 L.

  4. 4. 4. Interpret and bound

    State what the intersection means and whether it is permitted.

    You should see: At four minutes they match; four is in both domains.

Now change one thing

Use A(t)=5+3t and B(t)=17+t. At what t do they meet?

Your work

What to finish

An intersection calculation with both output checks and domain decision.

Common mistake

Watch for this

An algebraic crossing outside the model scope is not an established event.

How can I tell I understand it?

Distinguish an algebraic intersection from an event supported in the shared domain.

Try 3 questions

Quick knowledge check

Answer all 3 questions. Get at least 2 right and the lesson completes automatically.

Not yet passed

You’ll get an explanation for each answer. Signed in? Your results save to your account.

1Models 2+5x and 14+2x meet at which x?
2What is the shared output at the original intersection?
3A calculated intersection is at t=10 but both models only apply through t=8. What is defensible?
0 of 3 answeredApply the lesson to a short question, then review the explanation.
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Finish this lesson

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Answer all 3 questions above and get at least 2 right. You can review the explanations and try again.

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Sources and further reading

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OpenStax: systems by substitution
Source & access notes · checked 2026-10-01

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