Solution:
According to the question, the total number of packets is four and there are 5 post offices in all.
One parcel can be posted in five different ways, including at any of the two post offices. 5C1 is the result. Similarly, it can be posted in 5C1 ways for other parcels.
As a result, there are a variety of ways to send parcels via registered mail.
5C1 × 5C1 × 5C1 × 5C1 = 5 × 5 × 5 × 5
5C1 × 5C1 × 5C1 × 5C1 = 625 ways.