Final answer:
The z-scores that cut-off the middle 90% of the normal curve are -1.645 and 1.645.
Step-by-step explanation:
The z-scores that cut-off the middle 90% of the normal curve, which is represented by an area of 0.9000, are -1.645 and 1.645. These z-scores can be found using the z-table, which represents the area under the standard normal curve to the left of the z-score.
The area to the left of -1.645 is 0.05, and the area to the left of 1.645 is also 0.05. The difference between these two areas is the area between the z-scores, which is 0.9000.