Step-by-step explanation:
You start with milk : water = 7 :3
Total volume = 800 mL
Volume of milk = 7/10*800 = 560 mL
Volume of water = 3/10*800 = 240 mL water
Volume of milk must remain constant at 560 mL which is 2 parts
Therefore 3 parts water = 3/2*560 = 840 mL water final volume
You need to add 840 mL - 240 mL = 600 mL water .
Hope it helps
:DD
