If we go by general questions like these, your question is slightly incorrect. It should be "11 articles at Rs. 10" (not per article). Similarly "10 articles at Rs. 11" (not per article).
Now cost price of 1 article = Rs. 10/11
Selling price of 1 article = Rs. 11/10
So profit = 11/10 - 10/11 = 21/110.
profit% = $\frac{\left(\frac{21}{110}\right)}{\left(\frac{10}{11}\right)}*100 = 21\%$
For your exact question, the answer is simple, profit on one article = Rs. (11-10) = Re. 1
Profit % = $\left(\frac{1}{10}\right)*100 = 10\%$
In general, for these type of questions, find CP and SP of one item and find profit/loss accordingly.