Shortcut Trick
When the simple interest from different schemes is equal, the ratio of investments is inversely proportional to the product of Rate and Time.
Ratio of investments = 1/R1 : 1/R2 : 1/R3 (since time is 1 year for all)
Ratio = 1/4 : 1/5 : 1/10 = 5 : 4 : 2
Total parts = 5 + 4 + 2 = 11 parts = ₹2200 ⇒ 1 part = ₹200
Amount invested at 5% corresponds to 4 parts = 4 × 200 = ₹800
∴ The correct answer is ₹800.
Alternate Method
Total = ₹2200
Part 1
@ 4% for 1 yr
Part 2
@ 5% for 1 yr
Part 3
@ 10% for 1 yr
Interest is EQUAL across all three parts (I1 = I2 = I3)
Given:
Total sum invested = ₹2200
Rate of interest for scheme 1 (R1) = 4%
Rate of interest for scheme 2 (R2) = 5%
Rate of interest for scheme 3 (R3) = 10%
Time period (T) = 1 year
Formula Used:
Simple Interest (I) = (P × R × T) ÷ 100
Calculations:
Let the amounts invested in the three schemes be P1, P2, and P3.
⇒ P1 + P2 + P3 = 2200
According to the question, the simple interest earned from all three schemes is equal:
⇒ (P1 × 4 × 1) ÷ 100 = (P2 × 5 × 1) ÷ 100 = (P3 × 10 × 1) ÷ 100
⇒ 4P1 = 5P2 = 10P3
Dividing the equation by the LCM of 4, 5, and 10 (which is 20):
⇒ 4P1 ÷ 20 = 5P2 ÷ 20 = 10P3 ÷ 20
⇒ P1 ÷ 5 = P2 ÷ 4 = P3 ÷ 2
⇒ Ratio of investments (P1 : P2 : P3) = 5 : 4 : 2
Sum of the ratio parts = 5 + 4 + 2 = 11 parts
⇒ 11 parts = 2200
⇒ 1 part = 2200 ÷ 11 = 200
The amount invested at 5% is P2, which corresponds to 4 parts.
⇒ Amount invested at 5% = 4 × 200 = ₹800
∴ The correct answer is ₹800.
Additional Information
Equal Interest Condition in SI
If a total sum is divided into multiple parts such that the Simple Interest earned on each part is equal, the ratio of the principal amounts is directly given by: P1 : P2 : P3 = (1/R1T1) : (1/R2T2) : (1/R3T3).
Equal Amount Condition in SI
If a sum is divided such that the Final Amount (Principal + Interest) earned on each part is equal, the ratio of the principal investments is given by: P1 : P2 : P3 = 1/(100 + R1T1) : 1/(100 + R2T2) : 1/(100 + R3T3).
Simple vs Compound Interest Concept
In Simple Interest, the interest is calculated only on the original principal for the entire time period. In Compound Interest, the interest is calculated on both the principal and the accumulated interest from previous periods.