Calculate Percentage: Finding the Ratio of 161 White Cows in 800 Total

To solve this problem, we'll follow these steps:

• Step 1: Identify the part and whole of the sample.
• Step 2: Use the correct formula to calculate the percentage.
• Step 3: Match the calculated percentage with the given choices.

Now, let's work through the solution:

Step 1: Identify the given information:
The number of white cows (part) is $161$.
The total number of cows on the farm (whole) is $800$.

Step 2: Use the percentage formula:
Using the formula for percentage, we calculate:

$\text{Percentage of white cows} = \left( \frac{\text{Number of white cows}}{\text{Total number of cows}} \right) \times 100$

Substitute the known values:

$\text{Percentage of white cows} = \left( \frac{161}{800} \right) \times 100$

Perform the division: $\frac{161}{800} = 0.20125$

Multiply by 100 to convert to a percentage: $0.20125 \times 100 = 20.125\%$

Round to the nearest whole number: $\approx 20\%$

Step 3: Match this calculated estimate with the given choices:

The percentage of white cows in Marcelo's sample, and thus our estimated worldwide percentage, is approximately 20%.

Therefore, the solution to the problem, corresponding to choice 3, is $20\%$.