Auto- adjust cells to always add to a specific number/quantity

Hello Anyone,
I'm supposing this is the right place to ask this...I have virtually no knowledge of VBA and little of macros...I know a few basic formulas, which I thought were advanced before visiting this forum!

Here's my question:

I am trying to create a excel sheet for a friend, that has 3 or more cells which add to a specific number. Now, that is basic. What is needed, is that when data in at least one of the cells is changed, the other cells should automatically change to add to the specified number.

I will try to explain how he explained to me:

Let's say we have three products labelled "A", "B", "C".

My friend's job role includes product distribution, and he wants to simplify the process of loading a container (shipping container, 20', 40', 60', and so on). He wanted a spreadsheet that allowed him to manually enter in a quantity for Product "A", "B" or "C", and then the quantities for the other two products would automatically change, to always add to a manually specified amount.

For example:

Total number of products that can fit in container= 200,000
Product B quantity = 23,567

Product quantities for A and C should automatically change, to add to 200,000.

This will allow the sheet user to quickly see the maximum number of products "A","B" and "C" that can be loaded into the container for one shipment.

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Sorry, this is very confusing, and I have no idea how to write this in a sensible way!!
As I said, I have virtually NO knowledge of VBA and macros, so help would be greatly appreciated

PLEASE SOMEONE, HELP!!!! I'M IN UTTER CONFUSION!!!!

Thanks in advance!

Basic and simple but should give you an idea by looking at the cell formulas. Change the capacity of the different size containers in pink section for 20 40 & 60 foot container.
change the number of each product in the green and the resulting totals in the yellow.

Container Capacity.xlsx

hope it helps

Scott