Given that,
Amal $ \Rightarrow P=12000, R=8\%$, at $1$ year compound interest.
$\qquad \Rightarrow P=10000, R=6\%$, at semi-annually compound interest.
When interest is calculated semi-annually then, the rate will be half and time will be doubled.
Amal first $CI:$
$ \boxed {\text{Amount } = P \left ( 1+\frac{R}{100} \right)^T}$
$ \Rightarrow A=12000 \left (1+ \frac{8}{100} \right)^{1}$
$ \Rightarrow A=12000 \times \frac{108}{100}$
$ \Rightarrow A=12960$
$\because CI = A-P $
$\quad = 12960-12000$
$ \therefore \boxed{CI=960}$
Amal second $CI:$
$ \Rightarrow A=10000 \left (1+ \frac{3}{100} \right)^2$
$\Rightarrow A= 10000 \times \frac{103}{100} \times \frac{103}{100}$
$ \Rightarrow A=10609$
$\because CP=A-P$
$ \qquad=10609-10000$
$\therefore \boxed{CI=609}$
Total interest of Amal $=960+609=1569$
$\because$ Amal and Bimal get the same amount of interest.
Amal interest $=$ Bimal interest $=1569$
Bimal $\Rightarrow R=7.5\%$, at $1$ year simple interest.
Let the amount invested by Bimal $ = P.$
$ \boxed {SI= \frac{P\times R\times T}{100}}$
$ \Rightarrow 1569= \frac {P \times 7.5 \times 1}{100}$
$ \Rightarrow1569= \frac {P \times 75 \times 1}{100 \times 10}$
$ \Rightarrow P=20920$
$ \therefore$ The amount invested by Bimal $=20920.$
Correct Answer $:20920$