When is it advisable to use a full-copy sandbox?

The director of marketing has asked the app builder to create a formula field that tracks how many days have elapsed since a contact was sent a marketing communication. The director is only interested in whole units.
Which function should be used to calculate the difference?

To Synchronize Accounts, orders and shipments in real time, a developer has built a custom interface between an external system and salesforce, prior to deployment, the developer needs to confirm that the interface can sustain the syncing of thousands of records at a time. Which sandbox environment is recommended to complete performance and load testing?

All contact's mailing post code should match the account shipping postal code.How to do it?

Which statements are true regarding Roll-Up Summary Fields? (Choose three.)