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Calculators

Thirteen calculators, each built around one question rather than a general-purpose keypad. The trick is picking the one that actually matches what you are asking — several of these get reached for interchangeably even though they answer different things.

Money questions split three ways more often than people expect. A percentage calculator answers "what is X% of Y" and its reverse; a discount & tax calculator answers "what do I actually pay after both are applied"; and a loan & EMI calculator answers "what does this cost me every month for years." People often open the percentage calculator to work out a loan instalment, get a number that ignores compounding, and never notice it is wrong. If interest is involved over time, use the EMI calculator — plain percentage arithmetic will not account for the fact that early repayments are mostly interest.

Date questions have a similar trap. The date calculator handles "how many days between these two dates" and "what date is 90 days from now," including the fence-post question of whether the start or end day counts. The age calculator is a specific case of the same problem — years, months and days from a birth date — but it also has to decide what "one month later" means for a date like 31 January, which the general date calculator does not need to solve.

The tip calculator and fuel cost calculator both look like simple multiplication until rounding or units get involved. Splitting a bill four ways and rounding each share up is a different total from rounding the tip once — the tip calculator shows the difference rather than picking one silently. Fuel efficiency is worse: a jump from 8 to 10 km/L saves more fuel over a fixed distance than 20 to 25 km/L, despite an identical 25% improvement, because efficiency is a reciprocal quantity. The calculator proves this with a table rather than asking you to take it on faith.

For anything to do with pure number conversion — writing a figure out in words, converting to or from Roman numerals, or generating an unbiased random number — those are separate single-purpose tools rather than modes bolted onto the scientific calculator, which itself is for actual arithmetic and trigonometry, not formatting.