Subnetting math question.

Hi just a quick question i'm studying for my CCNA and have been trying to wrap my head around subnetting, as part of my study i've been pulling questions from online and one of the questions was :

identify the 125th subnet on a 10.0.0.0/18 (255.255.192.0)

and for hours now i've been trying to work out a quick way to work out the 125th but have no idea of easy ways.

i ended up doing:

10.0.0.0 is subnet 0

10.0.64.0 is subnet 1

10.0.128.0 is subnet 2

10.0.192.0 is subnet 3

10.1.0.0 is subnet 4

then i went "if subnet 4 is 10.1.0.0 then subnet 8 is 10.2.0.0 and so on"

and worked my way to subnet 125.

and my answer was 10.31.64.0

no idea if my answers are correct or not but can anyone point me in the right direction of an easy way to work out identifying specific subnet numbers?