Update multiple rows of a table with some calculation done with data from another table .

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SQL-Server

I have 3 table and my requirement is to update few records of a table(Using mathematical formula) with refence of the another table.
Sample table and sample data are as below:-

Filtered

ECode	CountyGroupname	Projectname	Worktype	Duration
101	NY	Proj1	WT1	10
101	NY	Proj2	WT2	20
102	NP	Proj1	WT1	20
101	NP	Proj3	WT3	10
102	NK	Proj1	WT1	20

NonProd

ECode	CountyGroupName	Duration
101	NY	10
102	NP	10
103	NY	10

Prod

ECode	CountyGroupname	Duration
101	NY	30
101	NP	35
102	ND	35
103	NY	40
104	GP	45

I want to update the "Filtered" Table Column Name is "Duration" as per the condition that:-
Update Filtered
Set Duration= (Formula will be:- Filtered.Duration * NonProd.Duration)/Prod.Duration when
Condition 1: Filtered.CountyGroupname=NonProd.CountyGroupname=Prod.CountyGroupName
Condition 2: Filtered.Ecode=NonProd.Ecode=Prod.ECode
where exists
()

Note:- As per the logic only first 3 Rows Of the duration in "Filtered Table" got updated.Rest of the field will not be changed.