Question: How to get a numerical answer using the erf function?

I didn't even know the erf function existed until doing this problem. I looked up how to use it, so I tried plugging in the explicity form into Maple, hoping it'd solve it, but it just spit back out the erf function.

I am trying to get a number answer.  A decimal. Because this is calculating a probability. How do I get Maple to give me a number here? Thanks!

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