Question 1143166
For the top 13% of lengths the z value is +1.127
z=(x-mean)/sd
1.127=(x-271)/7
x=271+7.889  or 278.9 days for minimum length

For the bottom 9% it will have a z=-1.34
The calculations will show 261.6 days 

on calculator, 2nd VARS 3 for inv-norm
type in decimal (so top 13% is 0.87 and bottom 9% is 0.09), then comma, then mean, then comma, then sd, then ENTER.