5. Place the numbers in descending order.

4.3 * 10 ^ - 5 6.4 * 10 ^ - 6 ; 4.3 * 10 ^ - 6 6.4 * 10 ^ - 4

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To place the numbers in descending order, we need to compare the exponents and the coefficients separately.

First, let's compare the exponents of the numbers:

4.3 * 10^-5 -> exponent = -5
6.4 * 10^-6 -> exponent = -6
4.3 * 10^-6 -> exponent = -6
6.4 * 10^-4 -> exponent = -4

Now, let's compare the coefficients (the numbers before the "* 10^") of the numbers:

4.3 * 10^-5 -> coefficient = 4.3
6.4 * 10^-6 -> coefficient = 6.4
4.3 * 10^-6 -> coefficient = 4.3
6.4 * 10^-4 -> coefficient = 6.4

Comparing the coefficients, we can see that 6.4 is larger than 4.3 for both numbers with exponent -4 and -6.

Now, comparing the exponents, we can see that -4 is larger than both -5 and -6.

Based on these comparisons, we can arrange the numbers in descending order:

6.4 * 10^-4
4.3 * 10^-5
6.4 * 10^-6
4.3 * 10^-6

So, the numbers in descending order are:

6.4 * 10^-4, 4.3 * 10^-5, 6.4 * 10^-6, 4.3 * 10^-6

6.4 * 10 ^ - 4 ; 4.3 * 10 ^ - 5 ; 4.3 * 10 ^ - 6 ; 6.4 * 10 ^ - 6

To place the numbers in descending order, we compare the values of the numbers. In this case, the numbers are written in scientific notation, where each number is represented as a decimal multiplied by a power of 10.

Let's reformat the given numbers:
- 4.3 * 10^(-5)
- 6.4 * 10^(-6)
- 4.3 * 10^(-6)
- 6.4 * 10^(-4)

Now, we can compare the exponents of the numbers. The smaller the exponent, the greater the value. So, we start by comparing the exponents first.

- 4.3 * 10^(-5) has an exponent of -5.
- 6.4 * 10^(-6) has an exponent of -6.
- 4.3 * 10^(-6) also has an exponent of -6.
- 6.4 * 10^(-4) has an exponent of -4.

When comparing the exponents, we find that the number with the smallest exponent is 6.4 * 10^(-6). So, it is the largest number.

Next, let's compare the values of the numbers with the same exponents.

- 4.3 * 10^(-5) has a value of 4.3.
- 6.4 * 10^(-4) has a value of 6.4.

Comparing these values, we see that 6.4 is greater than 4.3.

So, the numbers in descending order are:
- 6.4 * 10^(-6)
- 6.4 * 10^(-4)
- 4.3 * 10^(-5)
- 4.3 * 10^(-6)

Please note that since the two numbers with the same exponent (-6) are not directly comparable based on their values, they remain in the order as given in the question.