OE

What you need to do for 1990 and 1995 is find the per capita personal use, then find the percent decrease from 1990 to 1995. To do that, let’s plug in a value for the population of Country Y for 1990. Let’s use 100 million for the 1990s population. The per capita use in 1990 is of the total personal use, which is about 30 percent of 500 million or about 150 million. Set up 150 million kilowatt-hours, the total personal use, as a proportion with 100 million people, the population. The per capita use is 3/2or 1.5. Going on to 1995, you are told the population increased by 20 percent, so in 1995 the population was 120 million—using our initial figure of 100 million. What was the total personal use of energy? It was a little bit less than 20 percent of your total 600 million, so call it 20 percent of 600 million, or 120 million.

If total personal use is 120 million and there are 120 million people, that’s 1 kilowatt-hour per person. What’s the percent decrease? It’s a decrease of 1/3 or 33 1/3percent. But remember, in 1990, they were using a little more energy for personal use than you figured. The correct answer must be a little greater than 33 1/3percent, so 35 percent, (D), is the correct answer

Thanks Carcass, I totally ignored word 'personal' usage in the q stem.