Question : John invested a sum of money at an annual simple interest rate of 10%. At the end of four years, the amount invested plus interest earned is Rs. 770. The amount invested was:
Option 1: Rs. 650
Option 2: Rs. 350
Option 3: Rs. 550
Option 4: Rs. 500
Correct Answer: Rs. 550
Solution :
If the principal is p, then simple interest = $(770 - p)$
Principal = $\frac{\text{SI×100}}{\text{Rate×Time}}$
⇒ $p = \frac{\text{(770-p)×100}}{\text{10×4}}$
⇒ $2p = (770 – p) × 5$
⇒ $2p + 5p$ = 770 × 5
⇒ $7p$ = 770 × 5
$\therefore p$ = Rs. 550
Hence, the correct answer is Rs. 550.
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