Everyday math with the formulas shown, not hidden: percentages done correctly, EMI and SIP projections using the same formulas banks and fund houses use, GST in both directions, exact date arithmetic, and unit conversions with precise factors.
The finance calculators deserve one honest note: projections assume the rate you enter. The EMI figure is contractual; the SIP figure is an assumption about markets: treat them accordingly.
The conversions use exact factors rather than the rounded ones that circulate in tables, which is why a result occasionally disagrees with a figure somebody remembers. Data sizes are the clearest case: one gigabyte and one gibibyte are genuinely different quantities, storage is sold in the first and reported by operating systems in the second, and the converter shows both rather than picking a side.