An article was purchased from INR $1239$ including GST of $18\% $. Find the price of the article before GST was added.

Hint: First, assume the original price of the article before GST was added. Evaluate the $18\% $ of the original price and add it to the original price and equate to $1239$ and evaluate the price of the article before GST was added.

Complete step-by-step answer:
We are given that an article was purchased from INR $1239$ including GST of $18\% $.
We have to find the price of the article before GST was added.
Let the price of the article before GST was added be INR $x$.
Since the given price includes GST of $18\% $ therefore, evaluate the $18\% $ of the price before GST.
That is,
$18\% \,of\,x = \dfrac{{18}}{{100}} \times x$
(When we removed % sign we divide the number by $100$)
Write the expression in the decimal form.
$\dfrac{{18x}}{{100}} = 0.18x$
Now the article is purchased including GST it means we have to add the $18\% $ of the price before GST and the original price before GST.
Mathematically it can be written as,
$x + 0.18x = 1239$
Solve the equation to evaluate the value of $x$
$1.18x = 1239$
Divide both sides of the equation by $1.18$and evaluate the value of $x$.
$
  x = \dfrac{{1239}}{{1.18}} \\
  x = 1050 \\
 $
Hence, the price of the article before GST was added is INR $1050$.
Note: We can solve this by an alternate method which is shown below:
Let the price of the article before GST is added be $100$
It means the value of $1239$ would be $100 + 18 = 118$ as the price of the article was $1239$ when GST of $18\% $ was added.
Here, we have to evaluate the value of $100$.
$
  118 \to 1239 \\
  1 \to \dfrac{{1239}}{{118}} \\
  100 \to \dfrac{{1239}}{{118}} \times 100 = 1050 \\
 $
Hence, the price of the article before GST was added is INR $1050$.