On average, the pressure dropped by 0.83 bar each day.
From the Ideal Gas Law, you know that
The number of moles,
This means that, if the mass decreases by a certain factor, the pressure will decrease by the same factor.
Here, the mass has decreased to 10 kg from 15 kg, a factor of
The pressure will decrease by the same factor.
Hence, the new pressure will be
The drop in pressure is
This pressure drop occurred over four days.
∴ The average pressure drop per day was