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100 = ab^(-2)      ⇒ 100 =  a / b^2   (1)

.8 = ab^1    ⇒  .8 = ab   ⇒  .8/b  = a   (2)

Sub (2) into (1)  for a

100 =  .8 / b^3

b^3 =  .8/100

b^3 = (8/10)(1/100)

b^3  = 8/1000

b =  2 /10  =  .2

And

.8/.2 = a  = 4

So....the function is

y = 4(.2)^x

8000 is one standard deviation below the mean of 10400                        z-score = .1587

9000 is  1400/2400 =.58 standard deviations below the mean              z-score ~~ .2810

.2810 - .1587 = .1223          12 % of households are between 8000 and 9000 kwh consumption

I'll leave the graphing to you.....