The time it takes money to DOUBLE at simple interest has a one-line formula — no need to calculate the interest year by year.
Study Zone is free for our students — you just need to log in with your WhatsApp number first.
Log in with WhatsApp →The first two chapters of every book are free once you're logged in. To open this one, request an unlock and we'll get back to you on WhatsApp.
Simple interest is the same fixed amount every year — it never compounds on itself.
📌 SI = PRT ÷ 100
P = Principal (the original sum), R = Rate of interest per year (%), T = Time in years.
Worked example: P=₹5000, R=8%, T=3 years → SI = (5000×8×3)÷100 = ₹1200.
Amount = Principal + Simple Interest. Amount = 5000+1200 = ₹6200.
✨ Rearranging the formula
Find P
P = SI × 100 ÷ (R×T)
SI=₹900, R=6%, T=5yrs → P=₹3000
Find R
R = SI × 100 ÷ (P×T)
P=₹2000, SI=₹480, T=4yrs → R=6%
To double or triple your money, only the SIMPLE INTEREST needs to equal the principal (or twice the principal) — this gives a one-line shortcut.
📊 Shortcut formulas
| Goal | Condition | Formula |
|---|---|---|
| Double the money | SI = P | Time = 100 ÷ R |
| Triple the money | SI = 2P (amount = 3P) | Time = 200 ÷ R |
| Find the rate to double in T years | SI = P | Rate = 100 ÷ T |
Worked example: at 8% simple interest, money doubles in 100÷8 = 12.5 years, and triples in 200÷8 = 25 years.
Some problems give you the amount at two different times, and ask you to work backwards to find the rate or principal.
Trap: if a sum grows from ₹5200 to ₹5720 in 2 years, that ₹520 difference is the interest for 2 years only — find the ANNUAL interest first before finding the rate.
12 questions on simple interest, amount, and double/triple time problems.
