Question 1058647
The 80th percentile is p=0.8000 on the std normal curve
z=0.842
z=(x-mean)/sd
0.842=(x-3.5)/3
2.526=x-3.5
x=6.026 or 6.03 hours. That is 2.53 above the mean or 0.84 sd or 80th percentile.
more than 7 hours
z=(7-3.5)/3=3.5/3=1.167
for z>1.167, the probability is 0.1216 or 0.12.
It is more than 1 sd, above which is about 15% of the probability, so this makes sense.