Calculation when multiple records can be selected

[Pascal_Vallet]

I am working a field trip application to organize field trips for my school. Airtable is definitely game changer for this!
I met a problem on my budget tab:

• For each grade I have a total budget
  I can select the trip record from another TAB on a column named TRIPS for each grade
  The TRIPS cell can allow multiple records from the tab TRIP (if my grade 1 are doing 3 trips, we can select the three trips)
  I pull the total cost from all those trips.
  I need to show the remaining budget for each grade by a simple operation: budget - (total costs of all trips)
  But the problem is that the total costs for each trip are all in one cell. What should I do?

[ScottWorld]

You need to use a Rollup field instead of a Lookup field, with the formula SUM(values)

[Pascal_Vallet]

I’ve never used Roll up, I will look into it.