A20-g bullet collides inelastically with a 3-kg wooden block at rest. after the collision, the bullet-block system moves with a speed vf = 1.5 m/s. how much kinetic energy is lost in the collision?

I don't think it's possible to calculate it with the information given.

The kinetic energy after the collision is
  
              (1/2) (mass) (speed)²

         =  (1/2) (3kg + 20g) (1.5 m/s)²

         =        (1.51 kg) (2.25 m²/s²)

         =          3.3975 joules    .

That's nice to know, but we can't tell how much there was before the
collision, because we don't know what the speed of the bullet was.