mean and standard deviation

I have scanned over and over a similar problem on the forum, i just cannot understand.

I understand that i am to read the normal Z tables like a stem plot but cannot find any connection to the answers given.

Plastic buckets are made by machine on a production line. The weight of each bucket is normally distributed with mean 1250g and standard deviation 120g. Let X be the weight of a bucket in a randomly chosen batch.

*Mean- 1250 Std. dev = 120*

(a) Find Pr(X > 1250) This is the mean so prob has to equal half... (0.5)

(b) Find Pr(1250 < X < 1380)

= Find Z first z = 1380 – 1250 / 120 = 130/120 = 1.083

Pr(1250 < X < 1380) = look up z in tables

(c) Find Pr(1135 < X < 1275)

= Find Z (you need 2 values – lower should work out as -0.958 and upper as 0.208)tutor notes

= Pr(1135 < X < 1275) = look up Z values in tables and add together

It is spelled out for me, i cant see where to go. how to get the Z,