Can someone help me understand this subnetting question?

IP Address: 10.119.136.143/20

First useable host address: 10.119.128.1

I understand the subnet mask is 255.255.240.0 as it’s /20 (20 bits).

I understand that the first usable host address will have the final octet as .1, as .0 will be the network address.

What I really don’t understand is where the 128 octet comes from?