I Have a Table Like AdjustSize and its Descriptiotn is given Bellow;

SQL> Desc Adjustsize;
 Name                                      Null?    Type
 ----------------------------------------- -------- ------
 RECORD_NO                                 NOT NULL NUMBER
 SR_NO                                     NOT NULL NUMBER
 ITEM_SIZE                                          NUMBER
 ITEM_CLASS                                         VARCHA
 ITEM_GROUP                                         VARCHA
 ITEM_QUALITY                                       VARCHA
 BK_EXISTING                                        NUMBER
 BK_CAL_BASIS                                       VARCHA
 BK_PER                                             NUMBER
 BK_PROPOSED                                        NUMBER
 MTR_PER_TON                                        NUMBER
 FREIGHT_BK                                         NUMBER
 DISC_BK                                            NUMBER
 COIN_BK                                            NUMBER


Its Records is as:

SQL> Select * from Adjustsize where Record_no=34;

Record no  SR_NO  SIZE  Class  Group  Quality  BK_EXISTING  BK_PER   BK_PROPOSED  MTR_PER_TON  
---------  ------ ----- ----- ----   ------     --------    ------    ----------  ----------   
    34       1     100    L    BK     SS         361        -1000     351.5         106              
    34       2     100    L    BK     SS3        366          0       356.5         104          


Now  I Want To Calculate BK_ProPosed Column Value by Formula of 
 
    ROUND(((MTR_PER_TON*NVL(BK_EXISTING,0))+NVL(BK_PER,0))/MTR_PER_TON,2) 

But this Formuala is For Only Sr_no=1 and User Wants BK_Proposed of Sr_no=2 is Calculated by Sr_no=1 by 
 Adding 5 more

 I explane Breifly as

     Bk_Proposed of sr_no=1 is Equal to ROUND(((MTR_PER_TON*NVL(BK_EXISTING,0))+NVL(BK_PER,0))/MTR_PER_TON,2);

     Bk_Proposed of Sr_no=2 is Equal to (Bk_Proposed of Sr_no=1 + 5)
 

it is sure that Table Contain Only Two Records For a Record_no ;
 
How Can I Do by Using Trigger pls help me.