Writing Algebraic Equations
Express missing number problems algebraically using letters for unknowns; translate word problems into equations
What a learner can do afterwards
- Write 'I think of a number, double it, and add 5 to get 17' as 2n + 5 = 17
- Solve a one-step equation such as 3x = 24 and explain the reasoning
- Express a word problem as an algebraic equation and find the unknown value
The lesson
You already know how to write number sentences with a blank, like 5 + ⬜ = 12. In algebra, you swap the blank for a letter, such as n or x. That letter is called a variable, and it stands for the number you are trying to find.
Zara says, 'I think of a number, triple it, and subtract 4 to get 11.' Let n be her number. Tripling means multiply by 3, so the equation is 3n - 4 = 11. Add 4 to both sides: 3n = 15. Divide both sides by 3: n = 5.
To find the unknown, undo what was done to it. Addition undoes subtraction, and division undoes multiplication. Undo the last operation first.
Turn the words into a letter and symbols, then undo each operation to find the unknown number.
Watch it
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.