Q.An article is sold for ₹1,440 at a profit of 20%. Find its cost price.

A 20% profit makes the selling price 120% of the cost price, so divide the selling price by 1.20 to recover the cost price.

$CP=\dfrac{1440}{1.20}=₹1200$.

Here $SP=₹1{,}440$ and the profit is $20\%$ on the (unknown) cost price.

Step 1 — Relate SP to CP.

Step 2 — Solve for CP.

Check (independent verification). Profit $=1440-1200=₹240$ and $\dfrac{240}{1200}\times100=20\%$, as given.

Subtracting 20% of the selling price ($1440-288=₹1152$) is wrong, because the 20% is on the cost price, not the selling price. Divide by 1.20 instead.