As in Sales Cloud implementation, we can have custom rollup summary fields which can be done using trigger logic. Since its a rollup calculation it can be done using trigger logic on custom object A.
Relevant Skills and Experience
Experienced in Sales cloud, service cloud, marketing cloud using exacttarget, lightning system design.
Certified admin, pd1, pd2, appbuilder.
Experience above 6 years in salesforce implementation.
Proposed Milestones
$20 USD - After complete
Additional Services Offered
$10 USD - Deployment, data management.