Hello Bernat,
Thank you for your comment. You're right, I forgot to take into account the precision of the calculations. By allowing a 1% error, the question now works correctly.
For example, with an incorrect answer for the mass of 0.78 kg/m³, if the student answers that the mass of 2 m³ x 0.78 kg/m³ = 1.56 kg, the score is 0.5 for a set of partially correct answers.