thnx for responding, i just want to understand somthing:

when the  body (A) float, it affects on the liquid(b) by force (ma*g down) then the liquid affects it by an upward force (ma*g up) so they cancel each other so the remaining force which acts on measuring device will be the weight of the liquid (mb*g down ),

is that right or wrong?   

so how will it be (ma*g)+(mb*g) ??

thnx alot