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LorrMcLorrMc 

how to use process builder to mark earliest related record with tick box true as primary

I have been around the houses with this one but not really getting anywhere. 
I have a junction object Carer_Placement__c which has a lookup to the client contact and a lookup to the carer contact.
each Carer_Placement__c has a formula field Scotland_Client__c which indicates if a Scotland Client Yes/No. 
What I need to do is look at all Carer_Placement__c related to the Carer Contact and mark Carer_Placement__c which has the earlies Start_Date__c with Scotland_Client__c = Yes
I have created a tick box field on the Carer_Placement__c called First_Scotland_Placement__c which I hope to populate as true if the criteria above is correct. 
I have tried process Builder and Flows but it is just marking everything as true, please help I am really not good with flows or process builder so Do I need code to do this? 
Thank You in advance. 
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