Shortcut Trick
Interest for 2 years = 4644 − 4300 = ₹344
Interest rate per year = (344 ÷ 2) ÷ 4300 × 100 = 4%
Total interest for 5 years = 4% × 5 = 20%
Amount = 100% + 20% = 120% of Principal
Principal = (10104 ÷ 120) × 100 = ₹8,420
∴ The correct answer is ₹8,420.
Alternate Method
Given:
First case: Principal (P1) = ₹4,300, Amount (A1) = ₹4,644, Time (T1) = 2 years.
Second case: Amount (A2) = ₹10,104, Time (T2) = 5 years, Rate (R) is same.
Formula Used:
Simple Interest (SI) = (P × R × T) ÷ 100
Amount (A) = P + SI = P × [1 + (R × T) ÷ 100]
Calculations:
⇒ Interest (SI1) = A1 − P1 = 4644 − 4300 = ₹344
⇒ R = (SI1 × 100) ÷ (P1 × T1)
⇒ R = (344 × 100) ÷ (4300 × 2) = 344 ÷ 86 = 4%
⇒ Now for the second case, let the Principal be P2.
⇒ 10104 = P2 × [1 + (4 × 5) ÷ 100]
⇒ 10104 = P2 × [1 + 20 ÷ 100] = P2 × 1.2
⇒ P2 = 10104 ÷ 1.2 = 101040 ÷ 12
⇒ P2 = ₹8,420
∴ The correct answer is ₹8,420.
Additional Information
Simple Interest Property
In Simple Interest, the interest remains constant for every year as it is always calculated on the original Principal amount.
Effective Rate of Interest
The total interest percentage for 'T' years at 'R%' rate is simply (R × T)%. This is a very useful shortcut for solving Amount-related problems.
Relationship between A and P
Amount = Principal × [(100 + RT) ÷ 100]. This direct relation helps in finding the Principal when the Amount and Rate-Time product are known.