The Cobox — Knowledge is Growth
Tutorials, course notes and interview questions, free for everyone who wants to learn.
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How to calculate your exact age and the days between two dates
How age in years, months and days is counted, what happens with month ends and 29 February birthdays, how working days are counted, and how to use the Age & Date Difference Calculator.
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Percentages made easy: the four formulas and the Percentage Calculator
The four everyday percentage questions, percentage of a number, what percent, percentage change, and adding or removing a percentage, with worked examples and the traps that cause wrong answers.
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Fixed deposits vs recurring deposits: how interest and maturity are calculated
How FD and RD maturity values are worked out, the difference between cumulative and payout FDs, why an RD's first instalment earns the most, and worked examples with the FD & RD Calculator.
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New vs old tax regime in India: how the Income Tax Calculator compares them
How income tax is worked out under the new and old regimes for FY 2026-27, what the rebate, standard deduction, surcharge and cess do, and worked examples at several salaries.
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Compound interest explained: the formula, compounding frequency and the calculator
How compound interest differs from simple interest, what the formula means, how yearly, quarterly, monthly and daily compounding compare, and what effective annual rate tells you.
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How a SIP grows: the formula, step-up SIPs and the SIP Calculator
How a monthly SIP is estimated, what compounding does over 10, 15 and 20 years, why a yearly step-up changes the result so much, and what a calculator cannot promise about returns.
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How to calculate GST: add, remove, CGST, SGST and the GST Calculator
How to add GST to a price and take it out of a GST-inclusive price correctly, what CGST, SGST and IGST mean, why subtracting 18% gives the wrong answer, and examples at common rates.
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How EMI is calculated: the loan formula, worked examples and the EMI Calculator
The EMI formula explained step by step, a worked example for a home loan, how interest and principal change over time, and how tenure and rate affect the total you pay.
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Unit conversion made simple: metric, imperial, temperature and the Unit Converter
How length, weight, volume and temperature conversions really work, the exact factors behind them, why Celsius to Fahrenheit is not a single multiplier, and the US versus UK gallon trap.
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How to compress images without losing quality: WebP, JPEG and the Image Compressor
How image compression works, when to choose WebP or JPEG, what quality setting to start with, why resizing matters more than quality, and how to shrink photos privately in your browser.
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Lorem ipsum explained: where it comes from and how to use placeholder text well
The real origin of lorem ipsum, why designers use filler text, how much to generate for headings, cards and articles, and the mistakes that leave placeholder copy on live pages.
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Regular expressions for beginners: patterns, flags and the Regex Tester
A practical introduction to regular expressions: the building blocks, capture groups, flags, greedy versus lazy matching, patterns that waste time, and how to test everything live.