Compound Interest

Interest calculated on principal plus previously earned interest — grows faster than SI, often tested alongside it.

📖 ~4 min read 📐 RRB NTPC / Group D — Mathematics

Situation	Formula
Amount (annual compounding)	A = P(1 + R/100)ⁿ
Half-yearly compounding	A = P(1 + (R/2)/100)²ⁿ (rate halved, time doubled)
CI	A − P
CI − SI for 2 years	P × (R/100)²

💡 Example: CI on ₹10,000 for 2 years at 10% p.a. → A = 10000×(1.1)² = ₹12,100; CI = ₹2,100

💡 For half-yearly/quarterly compounding, always halve/quarter the rate and double/quadruple the time periods before applying the amount formula.

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