You're looking for a value 
such that
Because the distribution is symmetric, the value of in either case will be the same.
Now, because the distribution is continuous, you have that
The mean for the standard normal distribution is
, and because the distribution is symmetric about its mean, it follows that 
.
You can consult a score table to find the corresponding score for this probability. It turns out to be 
.
Because the distribution is symmetric, the value of
Now, because the distribution is continuous, you have that
The mean for the standard normal distribution is
You can consult a
