Hello, I am Juliana and is primary 6 as well. A disclaimer, I do not know if the answer I have presented here is certain and I suggest you ask your teacher, parent, or guardian to give you a more accurate answer or reason. Thank you. If you need an explanation, please reply to this message. I hope you have a nice Tuesday.

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 :)