Originally Posted by

**qazxsw11111** X~N(8,25)

P($\displaystyle {-a}\leq X \leq{a}$)=0.4

We need to find a.

I decided to use GC and standardize it to find a.

P($\displaystyle {(-a-8)/5}\leq {Z} \leq{(a-8)/5}$)=0.4

Since it is now standardized,

P($\displaystyle {(-a-8)/5}\leq {Z} \leq{(a-8)/5}$)

=1-2P($\displaystyle {Z}\geq{(a-8)/5})$=0.4

P($\displaystyle {Z}\geq{(a-8)/5})$=0.3

P($\displaystyle {Z}\leq{(a-8)/5})$=0.7

So I used Ti84 InvNorm Function to get $\displaystyle {(a-8)/5}$=0.5244

Which means $\displaystyle {a}$ is 10.622. However, the ans is 6.75.

Anyone can point out my mistake?

Thanks.