A man buys an article for 10% less than its value and sells it for 10% more than its value. His gain or loss percent is:________?

Let the article be of worth rupees x.

then, as per the statement, the cost price at 10% less value will be:

C.P=[(100-10)/100]*x

C.P=90x/100

C.P=9x/10 rupees

Sells at 10% more value means:

S.P= x*[(100+10)/100]

S.P=110x/100

S.P=11x/10 rupees

As 11x/10 > 9x/10, this means, man has gain percentage.

So, 

Gain=S.P-C.P

Gain=11x/10-9x/10

Gain=(11x-9x)/10

Gain=2x/10

Gain=x/5 rupees

But, we need gain%.

So,

Gain%=(Gain/C.P)*100

Gain%=[(x/5)/(9×10)]* 100

Gain%=(x/5÷9x/10)*100

Gain%=[(x/5)*(10/9x)]*100

Gain%=(2x/9x)*100

Gain%=(2/9)*100

Gain%=200/9

or

Gain%=22*(2/9)

So, Profit is more than 20%