Make algebra your unfair advantage.
Most algebra mistakes happen before the calculation. Turn the words into the right model, then choose the shortest route.
Name the unknown and its units
Write what x represents, including units. Translate 'per' into a rate and a one-time fee into a constant.
Choose your method
Use substitution when a variable is already isolated. Use elimination when coefficients line up. Graph intersections in Desmos when that is faster, but check the viewing window.
Solve for the requested expression
If the question asks for 2x + 3 and you have 4x + 6 = 18, divide by 2 to get 9. You do not always need x.
Check against the original
Substitute your answer into the original equation. Watch for negative signs, impossible lengths, and excluded values in denominators.
A membership costs $25 plus $18 per month. Model: C = 18m + 25. For a $115 budget, 18m = 90 and m = 5 months.