I have an important question I want to ask. I have already searched the web and my statistics book but I can not find the answer

I stumbled across this problem when preparing for my final from statistics and I can not figure out how we can get P=1 from a z-value=-10.6066...? Can somebody please explain?

There is also another problem following the same directions just with the change that its want the probability that its mean will be less than 970. There, the z-value turned out to be -13.59 and the P=0. My teacher checked this both answers as correct so I am very confused as to when a negative z-value equals 0 and when 1.

In a normal PD, the mean equals 1100 and the standard deviation equals 80.

If you choose a sample of size 50 from the original population, what is the probability that its mean will be more than 980?

z=(980-1100)/(80/√50)=(-120)/11,3137085=-10,60660172=-10,6066

P=1

If I choose a sample of size 50 from the original population the probability that its mean will be more than 980 is nearly certain.

Thank you so much for your help,

Kate