Plank T&G subfloor / girder repair in bathroom 48" OC
You may need to make a hole in the subfloor wall of your house to pass the new beam under the floor, there may be a conveniently placed vent grid.

replacment planks should probably span between girders.

Trim the rotted planks back flush with the side of the good girders
then fit a ledger board along the girder to support the ends of the new boards