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## Homework Statement

3 randomly selected observations form the standard normal distribution are selected. What is the probability that their sum is less than 2?

## Homework Equations

## The Attempt at a Solution

I know that the answer is 0.874928, but I don't know how to get that.

In my mind, you would say that 2/3=0.67, so what is the probability that the average of the 3 values is less than 0.67?

This gets me the wrong answer however.