I have tested with several accounts and its working, but I am still getting "challenge not yet complete... here's what's wrong: 
An update to an account record failed to update the mailing address fields of all child contact records. Make sure that the process is correct and that it is activated."

My process is activated and I have deactivated the previous validation rule that may affect it. I have gone through the record types and the profiles that use them etc.. where do I need to check please?
 

I am unable to complete the this module, need some help on the question
Create a formula field that determines the number of days between today and the last activity date for a case's account.

I created the formula in case object "Account.LastModifiedDate - NOW()"

but it throws an error  stating "Challenge not yet complete... here's what's wrong: 
The 'Days_Since_Last_Update__c' formula field did not return the correct number of days between an Account’s Last Activity Date and today"

Let me know if my approach is correct.
 

I am practicing on trailhead via using my developer org. The chalenging asked me to Create a Guest Administrator and deactivate it. I am currenly the System Administrator of my developer org. We have only one license for System Administrator. How should we solve this issue in order to:
Create a new user using the System Administrator profile and then deactivate that user to preserve the licenses in your org.The new user must use the System Administrator Profile.
The username for the new user must include 'guestadmin' somewhere in it.
The new user must be inactive.
I need some hints or advices. Thanks.