We know the probabilities are given by a normal distribution. Then
P(a within 1 sigma) = 68%
P(a within 2 sigma) = 95%
P(a within 3 sigma) = 99.7%
Here is a cool trick.
P(330 < a < 390) = (P(a within 3 sigma) - P(a within 1 sigma))/2
(see why? look at it from a geometric perspective.)
Then P(330 < a < 390) = (99.7-68)/2 = 15.85%
