Final answer:
The average length that fingernails grow in a year is about 3.5 centimeters or 1.38 inches, given the growth rate of 3 millimeters per month. This can vary with factors like age, gender, and health.
Step-by-step explanation:
The average length that fingernails will grow in 12 months is approximately 3.5 centimeters (1.38 inches). Considering that adult nails typically grow at a rate of about 1/10 of an inch, which is about 3 millimeters each month, we can explain that in one year, fingernails will grow roughly 36 millimeters.
However, this can vary based on several factors such as age, gender, and health. For instance, during some seasons, nails can grow faster, while in others they may grow more slowly. To achieve record lengths like those in the Guinness Book of World Records would take many years of continuous growth without cutting the nails.