Let X follow a normal distribution

mean=80 and Var=100

the probability is 0.08 that X is in the symmetric interval about the mean between which two numbers?

I did:

P( a <x< b)= .08

a= mean - k

b= mean + k

P( -k/stand dev < Z< k/st dev) =.08

F(k/st dev)- F(-k/st dev) =.08

F(k/st dev) - [ 1- F(k/ st dev)]= 0.08

2F(k/st dev)=1.08 from which k= 1 and a= 79

but the result on the book is a= 62.5

Help me, please. Thank you