You can just do an eval to create the new field. The only trick is that the field names within the eval statement will need to be in either single quotes or dollar signs to indicate to Splunk that you're referencing fields
| eval Average_time_taken='Total Time Taken'/'Total Records'
index=myindex sourcetype=mysourcetype "search string"
| stats sum(TotalRecords) As T, sum(TotalTime_Taken) As TT by Content
| eval Average_time_taken= TT / T
| rename T as "Total Records" , TT as "Total Time Taken", Content as Content_Type
| table Content_Type "Total Records" "Total Time Taken" Average_time_taken