I'm sorry to necro this thread but this problem is driving me nuts. I understand well and good how the Kaplan book and people in this thread arrived at the answer, but I'm not understanding how it is the correct answer. shridhar22 (and Kaplan) simply take 50% of the total post-efficiency usage and assume that half is the half that was using the efficient appliances. It is not. It is simply half of the total water usage after the reduction has been applied but proportionally has the exact same number of efficient users and inefficient users as the total.
52 billion gallons is arrived by taking 1/2 population using x water per capita and 1/2 population using 0.65x water per capita or:
p = population
x = per capita water usage
(0.5p * x) + (0.5p * 0.65x) = 52 billion gallons
The average water user in this scenario is using 35%/2 less than they would if nobody was using efficient appliances. That means without the efficient users total usage would be 17.5% more than stated, or 61.1 billion gallons for a total savings of 9.1 billion gallons.
Can someone take a look at this? Thank you.
