Need help with running calculation

Using the very helpful video instructions on this thread, I have been trying to build a running calculator about how students use their hours of the week. (This is based on one from another school, which we have permission to duplicate). I'd really like to make it work using "control loses focus" without the students needing to press a "calculate" button.

But I'm running into the problem where it just keeps adding up and up and I cannot wrap my head around how to replace the values in the calculation if the user changes one of their responses. 

Can anyone help? The file is attached.

Many thanks in advance!

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