Undoing Subtraction
Part of the Fundamentals section of Coddy's Math journey. Lesson 7 of 64.
In x - 4 = 9 the unknown has 4 taken away from it. Give it back to both sides:
x - 4 + 4 = 9 + 4
x = 13
Check by substituting: 13 - 4 = 9. True, so 13 is the solution.
A habit worth building now: after solving, put your number back into the original equation and see that both sides agree. It costs a few seconds and catches almost every slip. Later on it will catch something more interesting: answers that come out of the algebra but do not actually work.
Challenge
Solve the equation x - 6 = 15 for x.
Try it yourself
Solve for x
Test yourselfThis lesson includes a short quiz. Start the lesson to answer it and track your progress.
All lessons in Fundamentals
1What an Equation Is
The EquationFinding the UnknownThe Same to Both SidesSolved Means AloneRecap: First Equations4Two-Step Equations
Two Layers to UndoSubtract, Then DivideAdd, Then DivideA Division InsideNegatives in Two StepsRecap: Two-Step Equations2One-Step Equations
Undoing AdditionUndoing SubtractionUndoing MultiplicationUndoing DivisionInverse OperationsRecap: One-Step Equations5Tidying First
Like TermsTerms and Numbers MixedThe Distributive PropertyDistributing a SubtractionTidy, Then SolveRecap: Tidying First