if you want to find out how much he drank in total, its simple. Just add 0.75 and 0.8
¹0.75 Liters
+ 0.8 Liters
---------
1.55 Liters
(make sure to add the units AKA: liters)
Therefore,
Patrick drank 1.55 Liters of water in total.
Hope this helps and have a great day :)