4 answers
Hi,
If you want to have a sum of 'amount' field for all the related records whose record type is A then I suggest you do it using Rollup helper(an appexchange app). It's free.
Link: https://appexchange.salesforce.com/appxListingDetail?listingId=a0N30000009i3UpEAI
You cannot do this by simply creating a formula field.
If you don't want to use the app then the other way to do this involves a combination a process builder and a flow( a bit complicated).
You can refer to this link for help: https://www.linkedin.com/pulse/how-create-custom-roll-up-summaries-using-combination-mohammed-hassan/