Originally Posted by Cactus459

I don't know what rear derailleur you are using but I have a 40/30 crankset with a 11-36 cassette using a XT 10-speed mountain bike rear derailleur.
I do know if you are able to install a 10-42 and you can't use the 42t cog you can just adjust the limit screw on the rear derailleur to not travel to that cog.