Cogito
Grade 8 Math · Chapter 4 · Lesson 4
Writing Equations from Situations
Turning a story into something solvable.
10 problems · about 19 minutes · 8.EE.C.7
What this lesson teaches
The child writes and solves a linear equation modelling a real situation.
- Name the unknown before writing the equation.
- When two things are equal, each side of the equation is one of them.
- Check what the question actually asked for before answering.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
3 problemsReview — Equations with Brackets and Fractions: 5(x + 1) = 40. What is x?
Answer 7
Why x = 7.
Review — None, One, or Infinitely Many: 7x + 2 = 7x + 9. How many solutions?
Answer None.
Why No solution.
Review — Variables on Both Sides: Solve 7x + 3 = 4x + 18.
Answer 5
Why x = 5.
Warm Up
Straightforward practice. Get the method working first.
3 problems6 + 5m = 3m + 20. What is m?
Answer 7
Why m = 7.
15 + 2m = 31. What is m?
Answer 8
Why Take 15, halve.
5m = 3m + 12. What is m?
Answer 6
Why Take 3m from both sides.
Build It Up
The same ideas with more to keep track of.
2 problems8 + 4m = 12 + 2m. What is m?
Answer 2
Why Take 2m and 8 from both sides.
At m = 2, what is 12 + 2m?
Answer 16
Why 12 + 4.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
2 problemsThe Two Plans: 10 + 3m = 25. What is m?
Answer 5
Why m = 5.
The Cost: At m = 10, what does 20 + 5m cost, in pounds?
Answer 70
Why £70.