By what percent will the product of two numbers decrease if one of them is decreased by 20% and the other is decreased by 60%?

We have:

P1  = a x b

If a decreases by 20% and b decreases by 60%, then we have:

P2 = a x (100 - 20)/100 x b x (100 - 60)/100

     = a x 80/100 x b x 40/100

     = a x b x 8/25

=> P1 - P2 = ab - (8/25)ab = (17/25)ab

=> If a decreases by 20% and b decreases by 60%, product of a and b would decrease 17/25 = 0.68%

Hope this helps!

:)