Shortcut Trick
In Simple Interest, the interest earned is directly proportional to the time period.
Interest in 14 years = 3.5 − 1 = 2.5 times the Principal.
Time doubles from 14 years to 28 years, so the Interest also doubles.
Interest in 28 years = 2.5 × 2 = 5 times the Principal.
Total Amount = 1 (Principal) + 5 (Interest) = 6 times.
∴ The correct answer is 6.
Alternate Method
Given:
⇒ Initial multiple (n<sub>1</sub>) = 3.5
⇒ Initial time (T<sub>1</sub>) = 14 years
⇒ Final time (T<sub>2</sub>) = 28 years
Formula Used:
⇒ (n<sub>1</sub>−1)/T<sub>1</sub>=(n<sub>2</sub>−1)/T<sub>2</sub>
Calculations:
⇒ (3.5 − 1) / 14 = (n<sub>2</sub> − 1) / 28
⇒ 2.5 / 14 = (n<sub>2</sub> − 1) / 28
⇒ n<sub>2</sub> − 1 = (2.5 / 14) × 28
⇒ n<sub>2</sub> − 1 = 2.5 × 2 = 5
⇒ n<sub>2</sub> = 5 + 1 = 6
∴ The correct answer is 6.
Additional Information
Simple Interest Concept
Simple Interest (SI) is calculated only on the original principal amount. The formula is SI = (P × R × T) / 100.
Constant Interest Growth
In Simple Interest, the amount of interest earned remains constant for every equal interval of time if the rate and principal are unchanged.
Multiplier Rule for SI
If a sum becomes n1 times in T1 years and n2 times in T2 years, then (n1 − 1) / T1 = (n2 − 1) / T2.