Percentages
- To increase X by P% → X × (1 + P/100)
- To decrease X by P% → X × (1 − P/100)
- Successive percentage change of a% and b% → a + b + (ab/100)
Time & Work
- If A does a job in x days, A's 1-day work = 1/x
- Combined 1-day work of A and B = 1/x + 1/y
- Total time together = xy / (x + y)
Averages & Ratios
- Average = Sum of values ÷ Number of values
- If average changes when one value is replaced, difference = (new average − old average) × total count
- Ratio a:b, if total is T → parts are (a/(a+b))×T and (b/(a+b))×T
Who this is for
Candidates who've already covered these topics and want a fast pre-exam recall page, not a first-time explanation.