You can use the concept of proportions to estimate the number of defective watches in the total population.
First, calculate the proportion of defective watches in the sample:
Proportion of defective watches in sample = (defective watches in sample) / (total watches in sample)
Proportion of defective watches in sample = 3 / 400 = 0.0075
Next, use this proportion to estimate the number of defective watches in the total population:
Number of expected defective watches = Proportion of defective watches in sample * Total watches in population
Number of expected defective watches = 0.0075 * 10,500 = 78.75
Rounded to the nearest whole number, you would expect approximately 79 watches out of the total 10,500 to be defective based on this sample data.