There are some other Ideas solving the same problem like this, some suggested that traveling agencies should ask about the amount of food at the time of ticket purchasing, but in fact no one will be agree to reduce his/her food at service as they are fully paying for it and they don't know that at the time of meal they can eat more or less. It can work only in very rare case if some one has food related disease.
When other people (98%) open the full package of food, and leave the extra food in tray/box it starts to contaminate and deteriorate.
So we invented a sustainable solution that every person should get 50% of food and later on he/she can ask for other 50% after finishing first one.
So the another 50% of unopened packages of food can be preserve longer with good quality as they are not exposed to open air. This can save food and money.