Farmer Michael has 4 rabbits. The mass of Rabbit P is the same as the average mass of the 4 rabbits. Rabbit Q weighs 4 kg less than the average mass of the 4 rabbits. The mass of Rabbit R is 9 kg more than the average mass of the 4 rabbits. The mass of Rabbit S is 7 kg. Find the total mass of the 4 rabbits.
Let 1 u be the average mass of each rabbit.

Total mass of 4 rabbits
= 4 x 1 u
= 4 u

Mass of Rabbit P = 1 u
Mass of Rabbit Q = 1 u - 4
Mass of Rabbit R = 1 u + 9
Mass of Rabbit S = 7 kg

Total mass of the 4 rabbits
= 1 u + (1 u - 4) + (1 u + 9) + 7 
= 3 u - 4 + 9 + 7
= 3 u + 12

4 u = 3 u + 12
4 u - 3 u = 12

1 u = 12

Total mass of the 4 rabbits
= 4 u
= 4 x 12
= 48 kg

Answer(s): 48 kg