Finding a Missing Value
Part of the Fundamentals section of Coddy's Algebra journey. Lesson 57 of 64.
Often the letter you want is not the one on its own. Take this question: a car travels 120 km at 40 km per hour, and you want to know how long the trip takes. Substitute d = 120 and s = 40 into the distance formula d = st:
120 = 40t
The result is a one-step equation for t. Divide both sides by 40 to get t = 3, so the trip takes 3 hours. The formula itself did not change. You substituted the values you knew and solved for the letter that was left, the same way as with any equation.
One formula can answer three different questions. With d = st you can find the distance, the speed or the time, depending on which letter is unknown. You solve all three the same way: substitute the values you know, then solve for the letter that is left.
Challenge
MediumA rectangle has area 36 and length 9. Substituting into A = lw gives 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