Rewrite the Condition into the Block You Need
Some questions ask for an expression rather than the letters -- a ratio x/y, a sum over a product, a reciprocal sum -- and some systems hide the letters inside a repeated block such as 1/x. Instead of solving for each letter, reshape the given condition until it produces exactly that block and substitute it whole, or rename the repeated block as a new unknown and solve the simpler system it makes. Sometimes the value comes out though the letters are never pinned down; when the blocks themselves are solved, the original letters can be recovered from them.
grade 7–8 Expressions Convert to AlgebraIdentify Subproblems
Progression (8)
g7 1. Equal side products cancel the shared corner Number systemExpressions
g7 2. A relation fixes a ratio, so the expression is constant Expressions
g7 3. Rewrite numerators using the given total Expressions
g7 4. Turn the condition into x+y in terms of xy ExpressionsNumber system
g7 5. Clear denominators to reshape the condition Expressions
g7 6. Sum of reciprocals is sum over product Expressions
g8 7. Rename repeated reciprocals to get a linear system Expressions
g8 8. Times give equations in reciprocal speeds ExpressionsRatios