I think I follow what it is that you want to do. If I'm correct you want to summarize your data. I'm sure this is a duplicate but easy enough to explain. See if this does what you want. Right Click the field that has the A, BB, BE etc. and choose "Summarize". Then in the numeric fields of interest choose something like SUM, AVERAGE, MINIMUM, MAXIMUM etc. I think this will do what you are asking about. Hope it (and this screenshot) helps.
This will create a separate table. Your questions specifies populating the result in another field which doesn't make a lot of sense to me because the value will be duplicated for each "entity". If that is the desired result, you can join the output table back to the original and add it in (base the tabular join on the field you are calling the "entity". If you save the joined table using Data->Export (right click the layer in the table of contents) it will save the field(s) from the output table in the new copy of your original dataset.