Shortcut Trick
From the given conditions, R = 2.5P and R = 3Q. The ratio of investments P : Q : R is 6 : 5 : 15.
Let the investments in the three schemes be 600, 500, and 1500 units respectively.
Total interest in 1 year = 7% of 600 + 9% of 500 + 11% of 1500 = 42 + 45 + 165 = 252 units.
We are given total interest = ₹2520, so 252 units = 2520 ⇒ 1 unit = 10.
Amount invested in Scheme P = 600 units = 600 × 10 = ₹6000.
∴ The correct answer is ₹6000.
Alternate Method
Given:
Rate of interest for P = 7% p.a., Q = 9% p.a., R = 11% p.a.
Total Simple Interest (SI) earned in 1 year = ₹2520
Investment in R = 250% of P ⇒ R = (5 ÷ 2) × P ⇒ P ÷ R = 2 ÷ 5 = 6 ÷ 15
Investment in R = 300% of Q ⇒ R = 3 × Q ⇒ Q ÷ R = 1 ÷ 3 = 5 ÷ 15
Formula Used:
Simple Interest (SI) = (P × R × T) ÷ 100
Scheme P
P = 6x
R = 7%
SI = 0.42x
Scheme Q
P = 5x
R = 9%
SI = 0.45x
Scheme R
P = 15x
R = 11%
SI = 1.65x
Total Interest = 2.52x = ₹2520
Calculations:
⇒ Let the principal amounts be P = 6x, Q = 5x, and R = 15x.
⇒ SI from Scheme P = (6x × 7 × 1) ÷ 100 = 0.42x
⇒ SI from Scheme Q = (5x × 9 × 1) ÷ 100 = 0.45x
⇒ SI from Scheme R = (15x × 11 × 1) ÷ 100 = 1.65x
⇒ Total Simple Interest = 0.42x + 0.45x + 1.65x = 2.52x
⇒ We are given Total SI = ₹2520.
⇒ 2.52x = 2520
⇒ x = 2520 ÷ 2.52 = 1000
⇒ Amount invested in Scheme P = 6x = 6 × 1000 = ₹6000
∴ The correct answer is ₹6000.
Additional Information
Simple Interest (SI)
Simple interest is calculated only on the principal amount for the entire duration. Formula: SI = (P × R × T) ÷ 100.
Compound Interest (CI)
Compound interest is calculated on the principal plus any accumulated interest. Amount = P × (1 + R ÷ 100)T.
Equal Simple Interest Rule
If a sum is divided into parts P1, P2, P3 such that their simple interests are equal, their ratio is 1 ÷ (R1T1) : 1 ÷ (R2T2) : 1 ÷ (R3T3).