To determine the score for a baby that is 18 inches long, we can establish a scoring system based on the reference point of 20 inches.
- A baby that is 21 inches long receives a score of +1, which indicates that the score increases by +1 for every inch above 20 inches.
- A baby that is 16 inches long receives a score of -4, indicating a decrease in score for being below 20 inches.
The difference from the reference point of 20 inches to 18 inches is -2 inches.
- Since every inch below 20 corresponds to a decrease in score, we can conclude that the score decreases by the same amount.
For each inch below 20 inches:
- From 20 to 19 inches is -1 (from +0 for being at 20).
- From 19 to 18 inches is another -1.
Thus, starting from a score of 0 at 20 inches:
- 21 inches: +1
- 20 inches: 0
- 19 inches: -1
- 18 inches: -2
Therefore, a baby that is 18 inches long would receive a score of -2.
So the answer is: -2