Finding a Missing Value
Part of the Fundamentals section of Coddy's Math journey. Lesson 57 of 64.
Often the letter you want is not the one alone. "A car travels 120 km at 40 km per hour. How long does it take?" The distance formula d = st with d = 120 and s = 40 becomes
120 = 40t
a one-step equation for t: divide both sides by 40, t = 3 hours. The formula did not change; you substituted what you knew and solved for what was left, exactly as with any equation.
This is why the moves matter more than the formulas: one formula answers three different questions (distance, speed or time) depending on which letter is unknown, and the same solving skills answer all three.
Challenge
A rectangle has area 36 and length 9. From A = lw that is 36 = 9w. Solve for the width w.
Try it yourself
Solve for w
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