Let the mass of the white ball be x, the mass of the black ball be y and the mass of the heavy object be m.

2x + y= m ( 1)，3y + x= 2m(2)。

( 1)*2，4x+2y=2m。

(2) - ( 1)，y-3x=0，y=3x

So the mass of a black ball is equal to three white balls.