The latent heat of fusion of water is 334 J/g. How much energy is released when 20 grams of water at 0°C undergoes freezing?

The answer is: $Q = 6 , 68 k J$.
$Q = {L}_{f} m$
$Q = 334 \frac{J}{g} \cdot 20 g = 6680 J = 6 , 68 k J$.