# What's $P(z < −1.51\ {\rm or}\ z > 2.50)$ when $z$ is normally distributed?

Let $$z$$ denote a random variable having a normal distribution with $$\mu = 0$$ and $$\sigma = 1$$. Determine the following probability. (Round all answers to four decimal places.)

$$P(z < −1.51\ {\rm or}\ z > 2.50)$$

How does one approach this? I am unaware of what the OR means. I assumed it was a + but it appears to be the wrong answer.

• Your intuition about + seems right to me. What makes you think it isn't? You can write $\mathbb{P}(z<-1.51)=\Phi(-1.51)$ and $\mathbb{P}(z>2.50)=1-\Phi(2.50)$. – Ali Feb 14 at 1:30