Shortcut Trick
SI for 4 years (10 − 6) = 72,000 − 59,200 = 12,800
SI for 1 year = 3,200 ⇒ Principal (P) = 59,200 − (3,200 × 6) = 40,000
Original Rate (R) = (3,200 ÷ 40,000) × 100 = 8%
New Rate = 8% + 2% = 10%
New SI required = 76,000 − 40,000 = 36,000
Required Time = 36,000 ÷ (40,000 × 10%) = 9 years
∴ The correct option is (A).
Alternate Method
Given:
Amount after 6 years (A1) = Rs. 59,200
Amount after 10 years (A2) = Rs. 72,000
New Interest Rate = Original Rate + 2%
Target Amount = Rs. 76,000
Formula Used:
Simple Interest (SI) = (P × R × T) ÷ 100
Amount (A) = Principal (P) + SI
P
A1 (6 yrs)
59,200
A2 (10 yrs)
72,000
Diff = 12,800 (4 yrs)
SI/yr = 3,200
Calculations:
⇒ SI for (10 − 6) years = 72,000 − 59,200
⇒ SI for 4 years = 12,800
⇒ SI for 1 year = 12,800 ÷ 4 = 3,200
⇒ SI for 6 years = 3,200 × 6 = 19,200
⇒ Principal (P) = A1 − SI6 = 59,200 − 19,200 = 40,000
⇒ Original Rate (R) = (3,200 × 100) ÷ 40,000 = 8%
⇒ New Rate (R') = 8% + 2% = 10%
⇒ New SI for target amount = 76,000 − 40,000 = 36,000
⇒ Required Time (T) = (36,000 × 100) ÷ (40,000 × 10)
⇒ T = 360,000 ÷ 40,000 = 9 years
∴ The correct option is (A).
Additional Information
Effective Rate of Interest
When rate increases by r%, the additional interest earned is (P × r × T) ÷ 100.
SI vs CI Difference
For 2 years, the difference is P(R ÷ 100)2. For SI, interest remains constant every year.
Relation between Amount and Principal
A = P × [1 + (R × T) ÷ 100]. This can be used to directly find P if A, R, and T are known.