I'm looking at an example. There's one part that I don't understand.

The random variable Y has a normal distribution with mean 10 and standard deviation 2. Find the value of the constant a such that P(10-a<Y<10+a)=0.95.

Y~N(10,4)

Given P(10-a<Y<10+a)=0.95,

Therefore, P(Y<10-a)=0.025

10-a=6.080072028

a=3.92

What I don't get it is how does P(10-a<Y<10+a)=0.95

gives me P(Y<10-a)=0.025 ???

Any help is much appreciated. Thanks.