Need help with Calculating total time spent with user

I have two columns showing in my smartsheet: one stating - Start Time 08:20am and one for End Time 08:30am I also have another column titled Total Time Spent with User. What formula could i use to get the total time spent with a user? Thank you in advance

I have reviewed other comments on this, but for some reason the formula isn't working for me.

Any help with this would be appreciated!


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