Answers:
(a) 3.97 * 10^7
(b) 4.8 * 10^6
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Explanation for part (a)
The number over the year 2010 is 39.7 to represent "39.7 million people".
Replace "million" with "times 10^6" since 10^6 = 1,000,000.
So,
39.7 million = 39.7 * 10^6
39.7 million = (3.97*10^1) * 10^6
39.7 million = 3.97 * (10^1 * 10^6)
39.7 million = 3.97 * 10^(1+6)
39.7 million = 3.97 * 10^7
In short, we simply add 1 to the exponent for 10^6 to land on 10^7.
If we moved the decimal 7 spots to the right, then we'll go from 3.97 to 39,700,000 to help confirm we have the right answer. As you can see, scientific notation is useful to convey large numbers without having to write out all of the digits.
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Explanation for part (b)
Subtract the numbers over top the years 2010 and 2000.
39.7 - 34.9 = 4.8
It basically means
39.7 million - 34.9 million = 4.8 million
Then,
4.8 million = 4.8 * 10^6
If we moved the decimal point 6 spots to the right, then we go from 4.8 to 4,800,000 which helps confirm the answer.
Various online scientific notation calculators can also be used to verify the answers.