To clarify, , here both "amount" and "sum" are accepted as translations for "ποσό". In a previous sentence in this strengthen exercise, "σύνολο" was translated as "sum" and "amount" was rejected as incorrect. Is this correct that ποσό can be translated as either amount or sum but σύνολο cannot.
Ποσό can be translated as "sum" only for money: It was a large sum (amount of money) and Rachel realised she would have to borrow some money from the bank to pay it.=Ήταν ένα μεγάλο ποσό και η Ρέιτσελ συνειδητοποίησε ότι θα έπρεπε να δανειστεί χρήματα από την τράπεζα για να το πληρώσει. Σύνολο is "sum,whole(group), total".
Yes, I believe it is although I will let the experts chime in. I personally think of "σύνολο" as "summation" or "total". I hope this helps. :)