SOLUTION: I know this is a long question. I have done the best that I could do. I am having trouble figuring out the percentages. All of the question have something in common with each other

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Question 981583: I know this is a long question. I have done the best that I could do. I am having trouble figuring out the percentages. All of the question have something in common with each other.
1. According to the U.S. Energy Information Administration (www.eia.gov) the United States consumed 4.151 trillion kWh (kilowatt-hours) of electric energy in 2010. Write this number in correct scientific notation in terms of kWh.
My answer:5.151 X 10^12
2. Of this electric energy, 260.2 billion kWh were produced by hydroelectric
sources (think water over dams – like Snoqualmie Falls) in 2010.
a) Write this number in correct scientific notation in terms of kWh.
my answer:2.6 X 10^11
b) What percent of the total US electric energy did hydroelectric sources
account for in 2010? (Round to the nearest 10th of a percent.)
My answer: 62%
3. Additionally, 94.7 billion kWh were produced by wind power sources.
a) Write this number in correct scientific notation in terms of kWh.
My answer: 9.47 X 10^10
b) What percent of the total US electric energy did wind power sources
account for in 2010? (Round to the nearest 10th of a percent.)
I can not figure out the percentage.
c) What percent of the total US electric energy did hydro and wind power
sources account for in 2010 combined?
I can not figure out the percentage.
Found 2 solutions by solver91311, Boreal:
Answer by solver91311(24713) About Me  (Show Source):
You can put this solution on YOUR website!


1. Why is your answer 5.151 X 10^12 when the given significant digits are 4.151?

2a. Why did you round the significant digits in your answer to one decimal place? You carried three decimal places in your answer to #1.

2b. You divided the significant digits of the answer to 2a by the significant digits of the answer to 1. But the answer to 2a was a number times 10^11 and the answer to 1 was a number times 10^12. Your answer to 2b is off by a factor of 10.

3b. Move the decimal point on the significant digits on your answer to 3a 2 places to the left, making the number .0947 X 10^12. Now you can divide .0947 by 4.151 since the powers of 10 are now equal.

3c.

John

My calculator said it, I believe it, that settles it

Answer by Boreal(15235) About Me  (Show Source):
You can put this solution on YOUR website!
The first is 4.151 x 10^12
The second is correct.
The third is the ratio of 2.6 X10 ^11/4.51x10^12= 26X10^10/4.151x10^12 = 6.263x10^-2
That is 0.06263= 6.3%
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The fourth is
9.47 x 10^10/4.151 x 10^12=2.28 x 10^-2=0.0228 or 2.3%.
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The last is 3.549X10^11/4.151 x10^12, and it should be the sum of those two percentages (maybe slightly different because of rounding).
0.8550^10-1=8.550X10^-2, the latter being scientific notation.
That is 0.0855=8.6%.
Notice how 0.1 x 10^2=1 x 10^1
I multiply 0.1 x 10 and divide 10^2 by 10, which doesn't change anything.