Final answer:
The quality control engineer tests the parts every 180 seconds, which is equal to 3 minutes.
Step-by-step explanation:
To determine how often the quality control engineer tests the parts, we need to find the least common multiple (LCM) of the two times it takes for the machines to produce a part. The first machine makes a part every 12 seconds, and the second machine makes a part every 45 seconds. To find the LCM, we list the multiples of each number and find the smallest number that appears in both lists:
Multiples of 12: 12, 24, 36, 48, 60, 72, 84, 96, ...
Multiples of 45: 45, 90, 135, 180, 225, 270, ...
The LCM of 12 and 45 is 180. Therefore, the quality control engineer tests the parts every 180 seconds, which is equal to 3 minutes.