P(|X-80|<b)=0.9 X~N(80,15)

I literally have absolutely no idea of the rules of modulus. Apparently it goes to P(80-b<X<80+b) but I have no idea why... I've been searching the net for information and I've found nothing.

I'm very comfortable doing all normal distribution equations, I just don't understand how you get past the modulus part, as in how do you expand |X-80| to get something normal like P(X<b+80) which is easy to solve [not saying that is the right thing]

Thanks for your help