Answer:
1.6 L or 1600 mL
Step-by-step explanation:
You want to know the total volume of one 0.9 L box and two 350 mL boxes.
The units are related by ...
1 L = 1000 mL
Then the volumes of the given boxes are ...
0.9 L = 900 mL
350 mL = 0.35 L
You can add the volumes in either of the like units:
900 mL + 2 × (350 mL) = 1600 mL
0.9 L + 2 × (0.35 L) = 1.6 L
The total volume of the juice boxes is 1.6 L or 1600 mL.
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Additional comment
You can convert the units a couple of ways.
Because the fraction has equal measures in the numerator and denominator, it does not change the size of the volume. It only changes the units of the volume.