Three out of 200 machine parts were found to be defective.

about how many parts would be expected to be defective out of 2500 parts?

To answer the problem above, apply ratio and proportion. For every 200 machine parts, 3 are defective. The ratio that can be extracted from this is 200/3. Let x be the number of defective parts out of 2500 parts.
 
                                           (200 / 3) = (2500 / x)

From this equation, x equals 37.5. Therefore, there are approximately 38 defective parts.