Question:
A sells an article to B at a profit of 10% B sells the article back to A at a loss of 10%. In this transaction:

Option A):
A makes a profit of 11%
Option B):
A makes a profit of 20%
Option C):
A neither losses nor gains
Option D):
B loses 20%

Correct Answer is Option A):
A makes a profit of 11%

Explanation:
It could be easily shown by net percentage change graphic.
100(A)---10%↑--→110(B)-----10%↓--→99(A)
In this transaction A makes a profit of (110-99 = 11%) 11% .
[10% on selling to B and 1% profit on buying back from B]