The cost price of an article is 90% of its selling price. What is the profit or the loss as percent ?

Given:

The C.P. of an article = $90\%$ of its S.P.

Let the S.P. be ₹ $100$.

$\text{C.P.} = ₹ \dfrac{90}{100} \times 100\\[1em] = ₹ \dfrac{90}{\cancel{100}}\times\cancel{100}\\[1em] = ₹ 90$

(∵ S.P. is greater than C.P., means article is sold at profit.)

Profit = S.P. - C.P.

= ₹ 100 - ₹ 90

= ₹ 10

$\text{Profit\%} = \dfrac{\text{Profit}}{\text{C.P.}} \times 100\%\\[1em] = \dfrac{10}{90} \times 100\%\\[1em] = \dfrac{1}{9} \times 100\%\\[1em] = \dfrac{100}{9}\%\\[1em] = 11\dfrac{1}{9}\%$

Hence, Profit% = $11\dfrac{1}{9}\%$