plsql updatee a table value based on the sum of values from another table

I'm trying to update a table based on the sum of values from another table. The process I want to follow is:

1. select ColumnA,ColumnB from Table1 where id = 123
2. get total sum of the values in ColumnA and ColumnB from all returned records
3. update Table2's columnC with the total sum from above * 5 (or some value) where id =123

So if the return record from 'select ColumnA,ColumnB from Table1 where id = 123

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ColumnA ColumnB
1           5
3           0
1           7

And Table2's columnC would be set to (1+3+1+5+0+7) * 5 where id = 123

Thanks!

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You don't need PL/SQL for that.

UPDATE TABLE2 
SET  COLUMNC = ( SELECT (SUM(ColumnA + ColumnB))*5
                 FROM TABLE1 
                 WHERE id = 123 )