If $12:3::x:1$,find the value of $x$ and choose the correct option;
  {\text{A}}{\text{. }}\dfrac{1}{4} \\
  {\text{B}}{\text{. 1}} \\
  {\text{C}}{\text{. 4}} \\
  {\text{D}}{\text{. 5}} \\

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Hint: In this question we have to find the value of $x$, the key concept will be to simplify the given equation $12:3::x:1$ by using the concept of ratio and proportion to get the right answer.

Complete step-by-step answer:
We have been given that $12:3::x:1$………… (1)
Equation (1) will also be written as
$ \Rightarrow \dfrac{{12}}{3} = \dfrac{x}{1}$………. (2)
Now simplify the equation (2) we get,
   \Rightarrow \dfrac{{12}}{3} = \dfrac{x}{1} \\
   \Rightarrow x = 4 \\
Hence the value of $x$ is equal to 4.
And hence the option C is correct.

Note: Whenever we face such types of problems, we should try to solve these problems using the concept of ratio and proportion and after that using cross-multiplication to get the answer.
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