600ml=0.6litres. The fraction of orange juice to make 1.6litres of mixed fruit juice would be 0.6(volume of orange juice)/1.6(total volume of orange juice and apple juice). The fraction of orange juice needed for 4 litres of mixed juice would be proportionate to the fraction needed for 1.6 litres of mixed juice, hence you times 4 to get the correct volume. Hope my explanation wasn't too confusing :)