To calculate the combined population of North and South America for the year 2007, we need to add the populations given.
Let's denote:
- North America's population in 2007 as \( P_{NA} = 5.15 \times 10^x \)
- South America's population in 2007 as \( P_{SA} = 3.04 \times 10^y \)
To add these numbers, we should first ensure that both populations are represented with the same exponent.
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Assume \( x = 9 \) (for example): \[ P_{NA} = 5.15 \times 10^9 \] \[ P_{SA} = 3.04 \times 10^9 \]
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Adding the populations: \[ P_{total} = P_{NA} + P_{SA} = (5.15 + 3.04) \times 10^9 = 8.19 \times 10^9 \]
Hence, the combined population of North and South America in 2007 is approximately: \[ 8.19 \times 10^9 \text{ people} \]
Thus, the correct choice is:
B: 8.19 x 10^9 people