# Z values

#### bitterchocolate

##### New Member
hey i was wondering when calculating proportions of a normal distribution and they ask you what is the probability of selecting something between 60 and 70g.. when do we subtract the Z value of 70 from 60 and when do we add the Z values and subtract them from 1.. cause i was encountered two similar problems which we calculated differently.
thanks

#### JohnM

##### TS Contributor
I've made some corrections to my original post, after realizing an omission / error:

It sounds like you're looking for a standard procedure for doing these kinds of problems, and this is not advisable - it's better to get a visual understanding of the standard normal curve and what the question is asking:

(1) area above a certain value
(2) area below a certain value
(3) area between two values
(4) area outside of two values

Another complication is that z tables are different for different textbooks - some textbooks give you the area from negative infinity to z, other books give you only half the curve (0 to z) - but that's OK because the curve is perfectly symmetrical (however, most books give you negative infinity to z).

Generally, if the question asks for (1), you find the area below z and subtract it from 1. For (2), just look up the area below z. For (3), find the area below the larger z, then subtract the area below the smaller z. For (4), find the area below the larger z and subtract it from 1, then find the area below the smaller z, then add it to the area beyond the larger z.

However - make sure you have a visual understanding of the question and how it translates to a standard normal curve.

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