There is 125ml of orange concentrate in the jug. (We can tell because since there are 4 divisions between each 100ml, each small line must represent 25ml, and the orange concentrate is at the height of one small line above 100ml: 100+25=125.)
To find how much water to add, take 125ml from 500ml because the amount of water plus the amount of orange concentrate will equal the full 500ml:
