Shortcuts & write-ups.

maths · 20 Oct 2026

Successive percentage change, two ways

Price up 20%, then down 20%. Most students say "no change". Here's why that's wrong, the 10-second formula, and the ratio method that's even faster.

security · 18 Oct 2026

Race conditions, explained with a ₹500 wallet

How two requests sent at the same instant can spend the same money twice, and the simple fixes developers should use.