In metal forming operation, the true strain in the X-direction

In metal forming operation, the true strain in the X-direction is 0.3 and in the Y-direction –0.1. The true strain in the Z-direction is

Total volumetric strain = 0
ε_x + ε_y + ε_z = 0
0.3 - 0.1 + ε_z = 0
ε_z = - 0.2