Total mass = 8 kg
Mass of tool 1 = 4 kg
Mass of tool 2 = 2 kg
So,
Total mass = Mass of tool 1 + Mass of tool 2 + Mass of tool 3
=> 8 kg = 4 kg + 2 kg + Mass of tool 3
=> 8 kg = 6 kg + Mass of tool 3
=> 8 kg - 6 kg = Mass of tool 3
=> 2 kg = Mass of tool 3
