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Quick Reference Cheat Sheet
One core trick per topic, condensed for a fast refresher or for printing and sticking on the fridge. Click any topic for the full guide with examples.
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Mental Math
- Round one number to a friendly ten, add or subtract, then adjust back by the amount you rounded.
- To subtract, count up from the smaller number to the bigger one in easy jumps.
Multiplication
- ×9: fold a finger — fingers before = tens, fingers after = ones.
- ×4 or ×8: double the number 2 or 3 times in a row.
- ×5: multiply by 10, then cut the answer in half.
Division
- Turn it into a multiplication question: "__ times what equals this number?"
Fractions
- More pieces = smaller pieces (bigger denominator ≠ bigger fraction).
- Multiply straight across; for division, keep-change-flip.
Decimals & Percents
- Find 10% by moving the decimal one place left, then scale up or down from there.
Money Math
- Count coins biggest-value first. To make change, count up from the price to the amount paid.
Time & Measurement
- Every number on a clock = 5 minutes. For elapsed time, jump forward through whole hours instead of subtracting.
Geometry
- Perimeter = walk the edge and add. Area = rows × columns. Triangle area = (base × height) ÷ 2.
Word Problems
- Retell the story in your own words before touching any numbers. Break multi-step problems into a checklist.
Estimation
- Round every number, then calculate — use the rounded answer to sanity-check the exact one.
Everyday Math
- Divide price by size to compare deals fairly (unit price) — never compare totals for different-sized packages.