updating numeric fields: I keep failing.

Hi at all . I'm still sweating horses on my little project. I apologize in advance for the trivia I'm writing.

I'm in trouble because I keep climbing on a seemingly simple thing. I go into the details.

i have 3 tables. 

The first table has a list of vehicles. The fields are respectively: the key, the vehicle model, the total mileages of the vehicle.

The second table, is a list of people who can drive those vehicles.

the third table is where I process this information. During the compilation, when I enter the vehicle, I automatically get its total miles.
Finally, I register the miles of the end of the journey. I also get the miles driven.

The various arithmetic attempts to create a formula that modifies the initial miles of the vehicle in the first table, at the time of departure, fail.

I want to update the miles with those of the arrival. How should I proceed?

 

 

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