Day 1 is the start of a new billing cycle. Mailboxes m1, m2, and m3 are created in this billing period and hence will be charged for the entire billing period. Though the mailbox, m0, which was carried from the previous billing period is destroyed it will be charged because it was active at the begining of Day 1. The usage will be four because four mailboxes were active during the day.
The usage will be one because the new mailbox, m4 is created. The usage of the mailbox, m1 is already considered in Day1 and hence is not taken into account
The usage will be zero because no new mailbox is created. This is the last day of the current billing period.
Day 4 is the beginning of a new billing period. The mailbox, m4 is carried over to this current new billing period and is therefore charged in this billing period too. Mailboxes m1, m2, and m3, were created and destroyed in the last billing period and will not be charged in this billing period. Mailboxes m5, m6, and m7 are newly created in this billing period and hence will be charged for the entire billing period. The usage for Day 4 will be four because four mailboxes were active during the day.
|Day 27||Day 28||Day 29||Day 30||Day 1||Day 2|
Hence on Day 2, inventory will return 7 and the usages for 28, 29, and 30 will be ignored.
Hence on Day 1, collect will return 4.