Answer:
To find the cut-off lengths for nails to be discarded, you can use the fact that the mean of the nail lengths is 3 inches, and the standard deviation is 0.03 inches.
The bottom 2.87% of the nails are the shortest 2.87% of the nails, and the top 3.59% of the nails are the longest 3.59% of the nails. You can use the normal distribution to find the number of standard deviations from the mean that the cut-off lengths are.
Since the manufacturer intends to discard the bottom 2.87% and the top 3.59% of nails, you can use the standard normal table to find that the cut-off lengths are -1.28 standard deviations from the mean for the bottom 2.87% and 1.28 standard deviations from the mean for the top 3.59%.
To find the cut-off lengths in inches, you can multiply the number of standard deviations by the standard deviation in inches:
Low-end: -1.28 * 0.03 inches/standard deviation = -0.0384 inches
High-end: 1.28 * 0.03 inches/standard deviation = 0.0384 inches
Rounded to two decimal places, the cut-off lengths are:
Low-end: -0.04 inches
High-end: 0.04 inches
So the manufacturer should discard nails that are shorter than
3 inches - 0.04 inches = 2.96