What goes wrong
Check both sides of an equation, consistent units, and the same meaning for each letter. Variables lists the usual pitfalls.
Compare setups
Two different answers usually mean two different setups. Compare diagrams or equations line by line with this lesson and this one open for reference.
Typical fixes
You added 5 to the left side of an equation but not the right—both sides must stay equal. Or you added two ratios as if they were single numbers; ratios scale together, they do not add like plain fractions unless you rewrite them with a common structure.
Contrast
Wrong setup Corrected setup
Verify
Core lesson
Go deeper: Variables. Use the calculators below with your own loan or bill numbers, not only the examples on this page.