I need help understanding the formula for this because it is confusing for me. Thank you in adnvace.

Find the z scores that divide the normal curve in the following manner:

a.20% of scores in the middle and 80% in the tails

b. 25% in the middle and 75% in the tails

c. 70% in the middle and 30% in the tails

d. 90% in the middle and 10% in the tails

e. the 99th percentile (point at which 99% of the scores fall below)

f. the 90th percentile