I want to add a value to specific decimal field in one record, with the value in a decimal field in another record based on what is selected in another field.

I want to add a value to specific decimal field in one record, with the value in a decimal field in another record based on what is selected in another field.

I want to add a value to specific decimal field in one record, with the value in a decimal field in another record based on what is selected in another field.

example,...

project db, has the following fields, 

project number
elec design hours
mech design hours
technician hours 
assembly hours

time entry table has the following fields
Project number
job function (drop down with selection of elec design, mech design, technician design, assembly)
Total hours

I would like to update the proper field in the corresponding project in the project db with the sum of all the records in the time entry table that have the same project number and the same job function.