Adding value from One Table To Another And Update values

Question

I have two tables  A and B In Sql:

A

Id       Name      Weight

1         Wheat         20 kg

2          Rice             30Kg

 

 

B

 

  Name          Weight

  Wheat              10kg

 

I want to Add value of Wheat  from Table A to B and update values according Like that  

  After Adding 20kg  in B The value of Wheat In Table B would become 30kg  and Value of Wheat in A become 0...

Plz Kindly tell me What query Will apply in Sql to such Scenario