Answer : Option D

Explanation :

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Solution 1
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Let his investments be Rs.12000 and Rs.x
Rs. 12000 is invested at the simple interest rate of 10% per annum for 1 year
$MF#%\text{Simple Interest = }\dfrac{\text{PRT}}{100} = \dfrac{12000 \times 10 \times 1}{100} = \text{Rs. 1200}$MF#%
Rs. x is invested at the simple interest rate of 20% per annum for 1 year
$MF#%\text{Simple Interest = }\dfrac{\text{PRT}}{100} = \dfrac{x \times 20 \times 1}{100} = \text{Rs.}\dfrac{x}{5}$MF#%
$MF#%\begin{align}&\text{Total interest = Rs.}\left(1200 + \dfrac{x}{5}\right)\\\\ &\text{i.e., Rs.}\left(1200 + \dfrac{x}{5}\right)\text{ is the simple interest for Rs.(12000 + x) at 14% per annum for 1 year}\\\\ &\Rightarrow \left(1200 + \dfrac{x}{5}\right) = \dfrac{(12000 + x) \times 14 \times 1}{100}\\\\ &\Rightarrow 120000 + 20x = 14 \times 12000 + 14x\\\\ &\Rightarrow 6x = 14 \times 12000 - 120000 = 48000\\\\ &\Rightarrow x = 8000\end{align}$MF#%
Total amount invested = 12000 + x = 12000 + 8000 = Rs. 20000
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Solution 2
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If an amount P₁ is lent out at simple interest of R₁% per annum and another amount P₂ at simple interest rate of R₂% per annum, then the rate of interest for the whole sum can be given by
$MF#%\text{R} = \dfrac{\text{P}_1\text{R}_1 + \text{P}_2\text{R}_2}{\text{P}_1+\text{P}_2}$MF#%