How do I aggregate line totals in one file and send that total to a summary field in another file?

How do I aggregate line totals in one file and send that total to a summary field in another file?

Hi,

I have two forms, Enter_Transaction & Add_Distributions.  The ID field in the Enter_Transaction form corresponds to the Transaction_ID field in the Add_Distribution form.  There is a one-to-many relationship between the data in Enter_Transaction and Add-Distribution.

Enter_Transaction stores header information about financial transactions (e.g., a payment for 100 USD).  Add_Distributions stores line detail of distributions (e.g., 80 USD goes to a rent account and 20 USD goes to a utilities account). 

I want to sum the distribution line amounts (Line_Total field) by Transaction ID in the Add_Distributions form  and then pass this amount to the Distributed_Total field in Enter_Transactions. 

I am stumped how to do this.  I keep getting error messages when I try different types of code.  Can you help me?

Thanks so much,

Ann