A person purchased 11 articles for ₹ 10 and sells at 10 articles for ₹ 11. Find the gain percentage.

Cost price of 11 Articles = 10
Cost price of 1 Article = 10/11
and SP of 10 Articles = 11
∴ SP of 1 Article = 11/10
Now, gain = (11/10) - (10//11)
= (121 - 100)/110 = 21/110
And gain% = [(21/110) / (10/11)] x 100
= (21 x 11)/11 = 21