A technician has only two resistance coils. By using them singly, in series or in parallel, he is able to obtain the resistance $3,4,12$ and $16 \,ohms$. The resistance of the two coils are ........... $ohms$
A$6$ and $10$
B$4$ and $12$
C$7$ and $9$
D$4$ and $16$
Medium
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B$4$ and $12$
b (b)
$\frac{R_1 R_2}{R_1+R_2}=3$
$R_1+R_2=16$
$R_1 R_2=48$
$R_1\left(16-R_1\right)=48$
$R_1^2-16 R_1+48=0$
$R_1=4,12$
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