What is the final temperature when 100 g of water at 75#"^o#C is mixed with 75 g of water at 80#"^o#C?
These two systems will equilibrate. The system at a higher initial temperature will be exothermic, transferring heat to the contextually endothermic system.
To be sure, this is but a difficult representation of the familiar scenario,
where the heat supplied is merely coming from the exothermic system.