By what percent will the product of two numbers decrease if one of them is increased by 30% and the other is decreased by 30%?

Two numbers A & B, whose product is AB

when A increased by 30% and B decreased by 30%, the product is:
1.3A * 0.7B = 0.91AB
then
1AB - 0.91AB = 0.09 which is 9% decrease. So the answer is a 9% decrease. 
Hope this helps. :)