Person A takes 5 minutes in the shower.
Person B takes 20 minutes. They need to use mobility aids, a shower seat etc and cannot move quickly.
The water comes from a tank so there isn’t an endless supply. Person A says they should shower first as they’re quicker and will leave most of the tank water for person B.
Person B says they need to shower first as if they’re sat there for 20 minutes the water may go cold by the end of their shower if A has gone first and they’ll be sat under cold water and not able to move away from it quickly.