Answer:
The margin of error for this study is 2.5 pounds.
Step-by-step explanation:
The margin of error is the subtraction of the mean by the lower end of the confidence interval, and this must be equal to the subtraction of the upper end to the mean.
In this problem, we have that:
M - 13 = 18 - M
2M = 31
M = 15.5
The mean is 15.5 pounds.
So the margin of error for this study is 15.5 - 13 = 18 - 15.5 = 2.5 pounds.