Answer:
1. Mass of the blue ball is 5 kg. 
2. Mass of the red ball is 3.75 kg. 
Step-by-step explanation:
The following data were obtained from the question:
For the blue ball:
Force (F) = 15 N
Acceleration (a) = 3 m/s²
Mass (m) =.? 
Force the red ball:
Force (F) = the same with that of the blue ball = 15 N
Acceleration (a) = 4 m/s²
Mass (m) =.? 
1. Determination of the mass of the blue ball. 
Force (F) = 15 N
Acceleration (a) = 3 m/s²
Mass (m) =.? 
Force (F) = mass (m) × acceleration (a)
F = ma 
15 = m × 3
Divide both side by 3
m = 15/3
m = 5 kg
Therefore, the mass of the blue ball is 5 kg
2. Determination of the mass of the red ball. 
Force (F) = 15 N
Acceleration (a) = 4 m/s²
Mass (m) =.? 
Force (F) = mass (m) × acceleration (a)
F = ma 
15 = m × 4
Divide both side by 4
m = 15/4
m = 3.75 kg
Therefore, the mass of the blue ball is 3.75 kg