We need to evaluate the expression $\frac{0.42 \div 2.5}{0.5 \times 2.05}$ and express the result in standard form.

1. Problem Description

We need to evaluate the expression $\frac{0.42 \div 2.5}{0.5 \times 2.05}$ and express the result in standard form.

2. Solution Steps

First, evaluate the numerator:

$0.42 \div 2.5 = \frac{0.42}{2.5} = \frac{42}{250} = \frac{21}{125} = 0.168$

Next, evaluate the denominator:

$0.5 \times 2.05 = 1.025$

Now, divide the numerator by the denominator:

$\frac{0.168}{1.025} \approx 0.163902439$

Express the result in standard form:

$0.163902439 \approx 1.639 \times 10^{-1}$

3. Final Answer

C. $1.639 \times 10^{-1}$