A man sells two cows for Rs.15,640 each, gaining 15% on one and losing 15% on the other. Find his total gain or loss.

Rs.720 loss

Selling price of first = Selling price of second

Now ATQ, 

For first cow,

If, 115% of CP = 15640

Then, 100% of CP = 13600

For second cow, 

If, 85% of CP = 15640

100% of CP = 18400

Total CP= 18400 + 13600= 32000

Total SP = 15640 + 15640 = 31280

Loss = 32000 - 31280 = Rs.720 loss