Simple & Compound Interest: Shortcuts Beyond the Basic Formula

SI and CI carry 2-3 questions in every exam. While the basic formula is known to all, these shortcuts give you an edge.

The Difference Method

CI - SI for 2 years = P(R/100)². This single formula solves questions asking "find the difference between CI and SI" instantly without calculating each separately.

Effective Rate for CI

For 2 years at R%: Effective rate = 2R + R²/100. For 10% for 2 years: Effective = 20 + 1 = 21%. So CI on Rs. 1000 for 2 years at 10% = 21% of 1000 = Rs. 210.

Installment Formula

For annual installments: Each installment = Total amount / [sum of (1+R/100)^(-n) for each year]. For simpler calculation: If 2 equal annual installments of X pay off a principal P at R%: P = X/(1+R/100) + X/(1+R/100)².

SI Shortcut

If amount becomes N times in T years at SI: Rate = (N-1)/T × 100. Example: Amount doubles in 5 years → R = (2-1)/5 × 100 = 20%.