Ajar contains 0.85 liter of strawberry juice and 0.50 liter of mango juice. celina poured 0.15 liter of watermelon juice into the jar. she then drank 0.30 liter of the mixture.

You would add 0.85 liters + 0.50 liters + 0.15 liters = 1.5 liters. Then you would subtract, 1.5 liters -  0.30 liters = 1.2 liters.
The jar contains 1.2 liters after everything is added and subtracted.