This is what I was taught to call the "Rule of Three".

If 1 1/2 inches represents 45 miles then 1 inch represents 45/(1 1/2) miles. To simplify 45/(1 1/2) multiply top and bottom by 2 to give 90/3 = 30. So 1 inch represents 30 miles. Now 5 inches represents ...

Similarly, if 7 out of 42 bulbs are defective then that is a rate of 1 in 42/7 = 1 in 6 or 1/6 in 1. So out of 2682 bulbs expect (1/6) times 2682 = 2682/6 = ...