Answer:
(A) 4.7215×10⁹
Explanation:
You want the sum of 4.7×10⁹ and 2.15×10⁷.
Place value
As you know, a digit in a number has a value that depends on its location relative to the decimal point. The place value multiplier can be expressed as a power of 10. The first attachment shows place value multipliers for digit locations to the left of the decimal point.
Scientific notation
A number expressed in scientific notation has a power of 10 that is equal to the power of 10 for the place value multiplier of its most significant digit. That is ...
4.7×10⁹ = 4,700,000,000 . . . . "four point seven billion" or "four billion seven hundred million"
Similarly, the number 2.15×10⁷ is a representation of ...
2.15×10⁷ = 21,500,000 . . . . "two point one five ten-millions" or "twenty-one million five hundred thousand"
Addition (and subtraction)
As you know addition and subtraction are done on pairs of digits with the same place value. When we do these operations, we align the numbers so that digits with the same place value are added or subtracted from each other. For example, we add ones to ones, tens to tens, and so on.
We can shortcut writing all of the zeros of the expanded form of these numbers by doing the addition with a common factor:
4.7×10⁹ + 2.15×10⁷ = (4.7 +0.0215)×10⁹ = 4.7215×10⁹
The second attachment shows the result of using a calculator with the result formatted in scientific notation. Any scientific or graphing calculator, and any spreadsheet will accept input values in scientific notation, and will permit the output to be formatted the same way.